Hi all, my check engine light came on. I took it to Auto Zone, they hooked up there computer and the code P0420 came up Catalyst System Efficiency Below Threshold (Bank 1). Auto Zone said it was probably the oxygen sensor before the catalytic converter, so I replaced it. 3 days later it's back on, so I replaced the plugs. 2 days later it's back on :dunno: . This time I took it to Kragen Auto, they hooked the computer up and the same code was there, he reset my computer thru his handheld computer, 10 miles later it's back on. Ok, so I look online, OBD-codes.com and it says "the oxygen sensor downstream after the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs)". So, should I purchase the oxygen sensor after that converter instead of the one I already installed before the converter? Any help would be appreciated. :headbash:

Upstream = before the cat / Downstream = after the cat. Based on the assumption the vehicle doesn't have a ECM problem, I would suggest you try replacing the o2 sensor first; unless you have a high quality scanner that gives you the actual readings/values of the o2 sensor while the vehicle is running. This way you could tell if it's an ECM issue or actual o2 issue. Sometimes when the o2 sensor starts to go bad it becomes "lazy" and doesn't react quick enough to do it's job correctly...thus the code.

When you pull the sensor, if it's badly carbonized, you should also think about getting the throttle body cleaned and injectors flushed also... Preventive Maint...You should notice a difference esp if the vehicle has alot of mileage.

Hope this helps
:thmsup:

what year/model are we talking about here???? :dunno:

sounds like the egr port clogging problem that has plagued all of those cars. get the new egr pipe and drill out the port in the intake manifold with a 25/64ths drill bit. when that port gets clogged, it starts flagging all of the sensors behind it.

i just went through it myself.

there is a link on here if you search "EGR port" in a .pdf file that will show you how to do it. easy and a lot cheaper than the dealership.

If it's a '98-'02 there's a high likelihood of it actually being a cat. That's the code that says it!

An article found:

Before Checking a Catalytic
Converter DTC
Before you troubleshoot an OBD II vehicle that stores
DTC P0420 (67) (catalyst system efficiency below
threshold), run these quick checks:
1. Check for a leak in the exhaust system. If you find
one, repair it, clear the DTC, and test-drive the
vehicle.
• If the DTC doesn’t come back, return the vehicle
to the customer.
• If the DTC returns, go to step 2.
2. Connect the PGM Tester, and test-drive the vehicle
while an assistant monitors the voltage signal from
the secondary oxygen sensor (HO2S S2). After the
catalyst reaches operating temperature, the HO2S S2
voltage should stay between 0.5 and 0.8 V at steady
cruising speed. During deceleration, the voltage
should be steady at 0.1 V or less.
• If the voltage readings are OK, clear the DTC, and
return the vehicle to the customer.
• At cruising speed, if the voltage fluctuates or stays
below 5 V, go to step 3.
3. Measure the inlet and outlet external temperatures of
the catalytic converter with a thermometer capable
of reading up to 500°F.
• If the outlet temperature is more than 100°F hotter
than the inlet temperature, the converter is OK;
clear the DTC, and return the vehicle to the
customer.
• If the outlet temperature is less than 100°F hotter
than the inlet temperature, replace the converter.

Ahh geez, I guess I never said :withstupi It's a 1998 Accord LX Coupe 4cyl.

I also have the intermittent P0420 and already replaced both O2 sensors with good results, but not perfect results. Wonder if it's time to just replace the Cat, probably ... but this is a California car no longer in California ... the Cats I can easily buy locally LOOK the same as the stock Cat, but are they different in some way? It's a '98 L4 LX not LEV, ABS or anything special, plain L4... Can I use the "federal" Cat without modifications? And are the Autozone/Advance Auto cats, which are MUCH cheaper, OK to use vs. the Honda Cat with is hideously more expensive. I've got 105,000 miles, no warranty on the Cat, of course. Thanks.

Welcome pa0101!! :wave:

Why is the premature failure of cats so prevalent on 6th gen 4cyl's? Mine has 125K and the cat is still working good according to the smog test results *Knock on wood* Hope it doesn't go out too soon, otherwise I'll have to go to a hi-flo cat from Random Tech

One update question - has someone measured the in/out Cat temperatures as per the TSB posted earlier? I have a non-contact IR thermometer but only good up to 400 F, not 500 F ... wondering if I have a chance of catching the expected 100 F gradient?

And if "all" the checks per the TSB are OK, then what? Is there an ECU update to lessen the sensitivity? Is the real problem (as I have read in other postings/forums) that Honda became hypersensitive after the problems in the 5th gen?

I have a 99 Odyssey that also keeps setting that code at 125,000 miles, so I am running into difficulty preparing for state inspection. Some sources say it takes 40 warm-up cycles for the OBD-II to have sufficient data after a reset to pass the emissions test! A little research also shows premature setting of this code to be a common problem, although I'm not sure if it's just Honda that has the problem.

I wonder if those cheap $100 replacement cats will adversely affect the vehicle in some way? I would think the system is engineered a certain way by Honda and needs a certain backpressure, etc.

I have a 98 LX with the same P0420 problem. Have been confused by
many opinions on the web. I took the chance not to replace
the CAT but to replace the following:
1. Primary O2 sensor or before CAT - (Autozone) Bosch 13075 $97.41
2. Secondary O2 sensor or after CAT - (Autozone) Bosch 13071 $89.84
3. A new set of spark plug wires (Genuine Honda) $80.51
4. New distributor cap (Genuine Honda) $24.04
5. New rotor (Genuine Honda) $19.72
6. 4 new spark plugs (Genuine Honda) $12.73

Total cost: $324.25 (including tax in TX)

The CEL light went away after these new parts were replaced.
The car also passed the smog test the day after.

Like many Honda owner, the P0420 haunted me for almost 2-1/2 years as the
CEL comes and goes. I did not find any forum on the web that could provide
a "real" solution. Some opinions are misleading and simply waste
my time. I took pictures
during the whole process and I will try to
set up a web site to help those who have the similar problems.

I also cannot believe why you replaced just one. If one goes bad,
the other one goes bad at the same time. The one that I removed showed
that the secondary sensor was in a really bad shape, worst than
the primary.

Hi all, my check engine light came on. I took it to Auto Zone, they hooked up there computer and the code P0420 came up Catalyst System Efficiency Below Threshold (Bank 1). Auto Zone said it was probably the oxygen sensor before the catalytic converter, so I replaced it. 3 days later it's back on, so I replaced the plugs. 2 days later it's back on :dunno: . This time I took it to Kragen Auto, they hooked the computer up and the same code was there, he reset my computer thru his handheld computer, 10 miles later it's back on. Ok, so I look online, OBD-codes.com and it says "the oxygen sensor downstream after the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs)". So, should I purchase the oxygen sensor after that converter instead of the one I already installed before the converter? Any help would be appreciated. :headbash:

I have a 98 LX with the same P0420 problem. Have been confused by
many opinions on the web. I took the chance not to replace
the CAT but to replace the following:
1. Primary O2 sensor or before CAT - (Autozone) Bosch 13075 $97.41
2. Secondary O2 sensor or after CAT - (Autozone) Bosch 13071 $89.84
3. A new set of spark plug wires (Genuine Honda) $80.51
4. New distributor cap (Genuine Honda) $24.04
5. New rotor (Genuine Honda) $19.72
6. 4 new spark plugs (Genuine Honda) $12.73

Total cost: $324.25 (including tax in TX)

The CEL light went away after these new parts were replaced.
The car also passed the smog test the day after.

Like many Honda owner, the P0420 haunted me for almost 2-1/2 years as the
CEL comes and goes. I did not find any forum on the web that could provide
a "real" solution. Some opinions are misleading and simply waste
my time. I took pictures during the whole process and I will try to
set up a web site to help those who have the similar problems.

I also cannot believe why you replaced just one. If one goes bad,
the other one goes bad at the same time. The one that I removed showed
that the secondary sensor was in a really bad shape, worst than
the primary.

I have a 98 LX with the same P0420 problem. Have been confused by
many opinions on the web. I took the chance not to replace
the CAT but to replace the following:
1. Primary O2 sensor or before CAT - (Autozone) Bosch 13075 $97.41
2. Secondary O2 sensor or after CAT - (Autozone) Bosch 13071 $89.84
3. A new set of spark plug wires (Genuine Honda) $80.51
4. New distributor cap (Genuine Honda) $24.04
5. New rotor (Genuine Honda) $19.72
6. 4 new spark plugs (Genuine Honda) $12.73

Total cost: $324.25 (including tax in TX)

The CEL light went away after these new parts were replaced.
The car also passed the smog test the day after.

Like many Honda owner, the P0420 haunted me for almost 2-1/2 years as the
CEL comes and goes. I did not find any forum on the web that could provide
a "real" solution. Some opinions are misleading and simply waste
my time. I took pictures during the whole process and I will try to
set up a web site to help those who have the similar problems.

I also cannot believe why you replaced just one. If one goes bad,
the other one goes bad at the same time. The one that I removed showed
that the secondary sensor was in a really bad shape, worst than
the primary.

So did you end up replacing the Cat? I have the same thing going on but am not sure if I should replace the both sensors, one sensor, just the Cat, or all three.
It's hard finding a straight answer.

I DIDN'T replace the CAT.

It has been > 3 months now and no engine light since then
(nothing to do with the cel light bulb)

This 98 Accord is as reliable as before.

2006-06-07

Did you get better gas mileage after the O2 sensor replacements?

I have a 98 LX with the same P0420 problem. Have been confused by
many opinions on the web. I took the chance not to replace
the CAT but to replace the following:
1. Primary O2 sensor or before CAT - (Autozone) Bosch 13075 $97.41
2. Secondary O2 sensor or after CAT - (Autozone) Bosch 13071 $89.84
3. A new set of spark plug wires (Genuine Honda) $80.51
4. New distributor cap (Genuine Honda) $24.04
5. New rotor (Genuine Honda) $19.72
6. 4 new spark plugs (Genuine Honda) $12.73

Total cost: $324.25 (including tax in TX)

The CEL light went away after these new parts were replaced.
The car also passed the smog test the day after.

Like many Honda owner, the P0420 haunted me for almost 2-1/2 years as the
CEL comes and goes. I did not find any forum on the web that could provide
a "real" solution. Some opinions are misleading and simply waste
my time. I took pictures
during the whole process and I will try to
set up a web site to help those who have the similar problems.

I also cannot believe why you replaced just one. If one goes bad,
the other one goes bad at the same time. The one that I removed showed
that the secondary sensor was in a really bad shape, worst than
the primary.

I'm driving a 1999 Honda Accord V6. We just did the EGR port enlargement and this came up after a week of driving "P0420(Catalytic System Efficiency Below Threshold-Bank1)." It came on also a week after I changed the oil to Mobil 1 15W50 Extended Performance. My car has 200,000 miles on it with a rebuilt engine put in at 160,000. (My brother in law said I need a heavier oil for my car)So basically, the engine only has about 40,000 miles. I changed the oil to 5W30 yesterday after washing it with 4 1/2 quartz of 5W30 and a new oil filter placed.
Is Bank 1 divided into 2 sensors? The upper and lower 02 sensors? Should I replace both?
Any advise would be greatly appreciated.

At 200K you very well may have exactly what it says....bad CAT..... :dunno:

For a while I thought I was the only one experiencing the "Check Engine Light" problem. From this thread I guess not!

I have an Accord '98 EX that is giving the P0420 code as well. What's quite interesting is the fact that I replaced the cat just before the car hit the 60K (now it's ~ 94K). And I replaced it in a dealer! (it was still under warranty and didn't cost me a dime.) My question to the experts in this forum is: would it be possible that a factory cat would need to be replaced after only ~35K miles??

Also, from this thread, I guess that now I should focus on the O2 sensors. Would you agree?

Any help/suggestions will be greatly appreciated.

Thanks,

Rbfhome

I too just had a P0420 code set off my CEL... Within a month of buying the car too! I drove a Volvo for several years, so at this point, this isn't too much of an aggravation. Has anyone had any luck by putting in a non-OEM cat?

I am also considering switching out the stock exhaust for something discreet, but with a little character. This may be an age old question, but, is there a prefered discreet exhaust on this board? <on topic b/c Id do the cat and exhaust at the same time>

The check engine line (CEL) came on and we took it to Autozone to get a free code reading. It was P0420, which turned out to be a very popular code for all Honda Accords. We have a 1999 Honda Accord LX with 140K miles. We called several local Honda dealership and received an estimate of $1K to replace the catalytic converter (CV). This has to be done in order for our 1999 Honda Accord LX to pass the State of Texas annual inspection. We took our vehicle to a highly recommended muffler shop and they replaced an after market CV, with the total cost less than $300. It has been a year now and the CEL has not come on and needless to say, our vehicle passed the annual inspection.

I have done quite a bit of research on this P0420 code and there are a lot of good recommendations. I have tried all of the home-remedies and none seems to work for me (ie. add fuel additive chemical into the gas tank, rev up the engine at a lower gear on the
highway, erase the computer code, buy super unleaded gas, replace OS2 sensor, get a tune-up, replace spark plugs wires,
replace with Honda manufactured CV, and etc....)

This has proven to be an easiest method of fixing P0420.

Good Luck !

As Princess posted, the first things mentioned need to be checked, but also the car needs to be scanned with a Honda scanner to make sure the car is in proper fuel control, the trims need to be where they should be to ensure the cat can do it's job.

ok, as a short term "fix" buy a can of carb spray cleaner.
remove the intake duct leading to the throttle body.
use your hand to rev the engine and start spraying cleaner into the throttle body.
run nearly the whole can through it. make it run rough, but don't stall it.

Do this outside, the car will emit some horrendous smells and toxic fumes. The cat will begin to heat up severely as it burns up the unburnt carb cleaner and fuel. This usually works on cars that use oil, the engine oil, especially thrifted oils leave deposits on cats that poison them. If the cat's precious metal washcoat has become ineffective or the substrate has broken up, this will do no good.

When done, go out and beat the snot out of the car.

I've had some luck in keeping the P0420 away for a while this way.

I'd say on 99% of the cars once they start setting a P0420 the cat is dead or dying and needs to be replaced. The PCM in all newer cars are very, very good at what they do when it comes to catalyst monitoring. There are some cars that had too tight tolerance for that monitor and needed to be "desensitized" by having a reflash. The gen6 accord isn't one of them, neither is the Ody as far as I know.

Replacing air/fuel and the rear monitor o2 sensor won't do anything. If those sensors have an issue, normally they will set codes themselves for performance issues, i.e. slow response, etc.

When my Accord Check Engine light came on I changed the O2 sensor on the manifold (109 K). Shortly thereafter the light returned. I then changed the O2sensor on the cat converter and in about 10 days the light went out and has stayed out "knock wood" for the last couple years (134 K). The mileage was somewhat better before I changed the sensors but the performance was better after. I was told that you can sometimes smell the converter when it is going bad. A sulphur smell. Anyone heard of that? Seems likely that changing the sensors, although a bit costly, after 100 K is a common remedy. I uses Bosch after market but still cost about $180.

sulfur smell does NOT equal bad cat. Most cars will get the smell when the cat heats up....like a long uphill drive.

It IS a problem is the smell is inside though!!

Any opinion on using one of these devices to keep the CEL off: http://store.kteller.biz/product.asp?s=owgklf19701&strParents=&CAT_ID=0&P_ID=937&strPageHistory=search&numSearchStartRecord=1

It basically moves the O2 sensor away from the direct exhaust stream thus it doesn't sense all the HC's so it won't trigger a P0420 code. Obviously if your cat has had a melt down it won't help but if its just triggering the code now and then from old age, it may help you put off replacing the cat.

I have a Accord 1998 LX V6 with 175K Miles on it. The check engine light came on and I googled the problem. Read some posts on here and other websites and went to Autozone. The free code reading came up with a P0420. The next step was to check for a crack in the pipe before the CAT. Got it checked free and it was negative. Please note, if there is a crack, have it fixed first and reset the CEL and drive it around for a 100 miles before you move onto other troubleshooting steps. Then I shopped around for a CAT. Initially I bought a Universal Fit CAT for only $ 66 bucks and took it to Meineke. The guy there bitched about it not having a O2 sensor and he would have to do some cutting and welding etc and he could'nt guarantee it works!! So, I returned that and then bought a Universal Direct Fit from Autozone (make sure you also buy the gaskets) for $ 180/-. The Labor was $ 120 to fit the CAT and reset the CEL. Total cost $ 300/-. I drove the car for 100 miles and then took it for Inspection. It passed!!

The check engine line (CEL) came on and we took it to Autozone to get a free code reading. It was P0420, which turned out to be a very popular code for all Honda Accords. We have a 1999 Honda Accord LX with 140K miles. We called several local Honda dealership and received an estimate of $1K to replace the catalytic converter (CV). This has to be done in order for our 1999 Honda Accord LX to pass the State of Texas annual inspection. We took our vehicle to a highly recommended muffler shop and they replaced an after market CV, with the total cost less than $300. It has been a year now and the CEL has not come on and needless to say, our vehicle passed the annual inspection.

I have done quite a bit of research on this P0420 code and there are a lot of good recommendations. I have tried all of the home-remedies and none seems to work for me (ie. add fuel additive chemical into the gas tank, rev up the engine at a lower gear on the
highway, erase the computer code, buy super unleaded gas, replace OS2 sensor, get a tune-up, replace spark plugs wires,
replace with Honda manufactured CV, and etc....)

This has proven to be an easiest method of fixing P0420.

Good Luck !

So I like everyone else on this forum have the infamous P0420 code. I replaced my Catalytic Converter (CAT) about 2 years ago and it has been about 25,000 miles tops since the replacement. At this point in time should I go ahead and replace the oxygen (O2) sensors? I have a 1997 Honda Accord LX 2.2L, 5 speed. I don't think the O2 sensors have been replaced at all. Has anyone had to replace anything else besides the O2 sensors and/or CAT?

I have a Accord 1998 LX V6 with 175K Miles on it. The check engine light came on and I googled the problem. Read some posts on here and other websites and went to Autozone. The free code reading came up with a P0420. The next step was to check for a crack in the pipe before the CAT. Got it checked free and it was negative. Please note, if there is a crack, have it fixed first and reset the CEL and drive it around for a 100 miles before you move onto other troubleshooting steps. Then I shopped around for a CAT. Initially I bought a Universal Fit CAT for only $ 66 bucks and took it to Meineke. The guy there bitched about it not having a O2 sensor and he would have to do some cutting and welding etc and he could'nt guarantee it works!! So, I returned that and then bought a Universal Direct Fit from Autozone (make sure you also buy the gaskets) for $ 180/-. The Labor was $ 120 to fit the CAT and reset the CEL. Total cost $ 300/-. I drove the car for 100 miles and then took it for Inspection. It passed!!

It has been a 1000 miles and less than a month since I passed the Inspection and the Check Engine light came on again. I took it to the Auto Zone and the guy said, I need to replace the O2 Sensor!! Has anyone faced this problem? Since I don't have to take in my vehicle for inspection for another 2 yrs, should I just let it be without trying to replace the O2 Sensor?

When my Accord Check Engine light came on I changed the O2 sensor on the manifold (109 K). Shortly thereafter the light returned. I then changed the O2sensor on the cat converter and in about 10 days the light went out and has stayed out "knock wood" for the last couple years (134 K). The mileage was somewhat better before I changed the sensors but the performance was better after. I was told that you can sometimes smell the converter when it is going bad. A sulphur smell. Anyone heard of that? Seems likely that changing the sensors, although a bit costly, after 100 K is a common remedy. I uses Bosch after market but still cost about $180.

It makes me feel that I replaced the wrong part!! I had actually replaced the CAT and got my Inspection passed and the light returned within a month after the Inspection! Maybe I shoulda started with the O2 Sensors first and may not have to replace the CAT!!

This problem ****s! I have had two cheap Midas cats and now the code is set again (99 Odyssey, so far so good on the 02 Accord with 120k). I don't know what the answer is but it sure is frustrating and expensive for a LOT of people. The time, effort, and money wasted on this dubious issue is HUGE.

How about the rear 02 sensor?
Check life data, look for FI Temp sensor (forgot honda jargon name) should be 180-190F (Mine is 182-188F).

As Princess posted, the first things mentioned need to be checked, but also the car needs to be scanned with a Honda scanner to make sure the car is in proper fuel control, the trims need to be where they should be to ensure the cat can do it's job.

ok, as a short term "fix" buy a can of carb spray cleaner.
remove the intake duct leading to the throttle body.
use your hand to rev the engine and start spraying cleaner into the throttle body.
run nearly the whole can through it. make it run rough, but don't stall it.

Do this outside, the car will emit some horrendous smells and toxic fumes. The cat will begin to heat up severely as it burns up the unburnt carb cleaner and fuel. This usually works on cars that use oil, the engine oil, especially thrifted oils leave deposits on cats that poison them. If the cat's precious metal washcoat has become ineffective or the substrate has broken up, this will do no good.

When done, go out and beat the snot out of the car.

I've had some luck in keeping the P0420 away for a while this way.

I'd say on 99% of the cars once they start setting a P0420 the cat is dead or dying and needs to be replaced. The PCM in all newer cars are very, very good at what they do when it comes to catalyst monitoring. There are some cars that had too tight tolerance for that monitor and needed to be "desensitized" by having a reflash. The gen6 accord isn't one of them, neither is the Ody as far as I know.

Replacing air/fuel and the rear monitor o2 sensor won't do anything. If those sensors have an issue, normally they will set codes themselves for performance issues, i.e. slow response, etc.

Not in my case. My P0420 was the rear sensor...

Things checked:
My CAT was running a 120F difference between in/output (a short 10 minute drive was done for the worm up).
A smoked up the tail pipe test, checked for leaks. None found.
Both front and rear sensors (voltage) moved more than expected. Rear seemed to me to be slow. Rear should not move much. The rear was somewhat matching the front. Now (rear) is ~0.7V (I forgot), stays steady now...

May have been originally effected by a lazy thermostat, as my car was running at 150-180F, mostly at 168F. After replacing the thermostat its now 182-188F.

In total, I replaced the:
Thermostat, it was bad, it was sticking open sometimes.
Front sensor, the mechanic thought it mite be bad. I thought it was the rear sensor. It had no effect once replaced.
Rear sensor. No MIL for over one month...

Nither front or rear sensor, failed any test. However, the rear one was bad...

Hello my name is Jason and I'm new to this forum. I was surfing the net looking for the engine code for the 2010 accord coup 4cyl. While doing so I stumbled upon this thead and was intrigued. Mainly because I was having somewhat of the same problem with my 05 Scion tC that I recently got rid of. The only difference was I knew why and what the problem was. For me it was because I change the header out for performance and as soon as I did I threw the same code. when that code came up it not only said bank1 was high but bank 2 was high and vice versa. to fix the problem I had to put this in:

http://www.robearracing.com/pd-megan-racing-angled-02-simulator-cel-fix.cfm

I only put 1 in on bank 2 and that cleared up the problem with bank 2.

Now my question to you guys is did this problem start happening with your stock header/s or did you guys change them out for aftermarket.

I was stock. If your loosing heat, it's possible to have CAT efficiency problems. Rapping the header, probably would help...

So U replaced the rear sensor too???

99 accord ex v6

Recognizing this thread is old, I had the P0420 for a brief period (1 week maybe), then the P0401 (I think) code for a while. I asked the dealer to check it out and they found nothing wrong with the cat. So I went ahead and did the EGR service myself, took off the intake, cleaned out the pipe, etc. Car passed the state inspection, but then 2 days later and probably not more than 100 miles. The P0420 code returned. At this point not sure what to do.

I did put a fancy scanner on the car and plotted the output of the O2 sensors. It seems the downstream sensor is tracking the upstream sensor closely. This leads me to think the cat is doing nothing. I just spent $2500 to have the tranny and axles replaced at the dealer so I still have a 3 yr warranty on that.

Any thoughts? If I go to replace the cat, the dealer is spendy ($1000) but I can buy the honda cat online for $700 or so. Is it easy enough to myself?

Thanks

I have a 2001 Accord LX 2.3 4 cyl 200k miles with check engine light on. It will not pass smog. Engine code P0420. I was told it could be the Cat, or one of the two oxygen sensors. I replace the two Oxygen sensors because that was cheaper than a new Cat. No Change. The dealer wanted $1,400 for the cat. I found a local muffler shop who install a new after market cat with warrenty for $230. That solved the problem. Dealer told me code P0420 is always the cat.

I have a 2001 Accord LX 2.3 4 cyl 200k miles with check engine light on. It will not pass smog. Engine code P0420. I was told it could be the Cat, or one of the two oxygen sensors. I replace the two Oxygen sensors because that was cheaper than a new Cat. No Change. The dealer wanted $1,400 for the cat. I found a local muffler shop who install a new after market cat with warrenty for $230. That solved the problem. Dealer told me code P0420 is always the cat.
That's what they think. BTW, the replacement CAT you got is likely not the correct type CAT, unless you have LEV engine. Did you pull fuse 13 or erase the stored code? The CAT is that expensive because its different that 99% CAT's out there. How a SMOG tech would know, weather a car has the right CAT, I dont know...
Good luck!

Hey guys,
I'm glad I found this thread, know it's old too, but sadly my 6th gen experienced the CEL today. I've notice it before. Usually it blinks 3-4 times after I start the car, but it would just go back to normal. Though today is totally different and got me depressed. Turned on my car and it acted funny. The CEL won't turn off this time. It felt like it was having a hard time and sounded like the engine was gonna die. I turned on the AC and it started chugging, so I turned it off right away. When I pressed on the break and gas pedals, they felt very stiff. When I finally drove it, it felt like the car was having a hard time with power. Usually pushing the gas pedal a lil, the car would just excel lively but today she felt weak =[. The trembles were harder this time, and I was soo scared it was gone die in the middle of the road. After I got to the bank and started her again to get back to work, she seemed a lil bit ok, but the CEL was still on. She still felt weak with excelling and I didn't wanna drive her over 35MPH. So does this mean I have the dreaded P0420 code as well =,[?? I can't seem to understand it though cause she's only at 80+k.......

Get your code read and then we can discuss something.

Hey guys,
I'm glad I found this thread, know it's old too, but sadly my 6th gen experienced the CEL today. She still felt weak with excelling and I didn't wanna drive her over 35MPH. So does this mean I have the dreaded P0420 code as well...

Definitely scan it - you can pick up an OBDII reader pretty cheap. What does it do at idle? If it stumbles and shudders, and then continues to shudder just off idle when you press the gas pedal, it could be the upstream O2 sensor. If, though, it feels like nothing is happening when you try to accelerate, the cat could be breaking down inside and plugging up - if that is the case, eventually it will worsen to the point where you won't even be able to keep up at highway speeds (ask me how I know! Our accord went from fine to no faster than 45 mph over one weekend).

But the codes are what you want next. If you don't want to get a reader, the popular auto stores should hook you up for free.

Continuing the topic - we've got an '00 accord ex with the LEV 4-cyl. Replaced the front O2 sensor about 4 years ago (we got the p0420 and whatever the front sensor code was that time, along with driveability problems), then about a year and a half ago the cat self-destructed and was replaced (P0420 as well as the symptoms I referred to above).

Now I just got a CEL tonight - P0420 again, but no obvious driveability problems. I think I'm going to start with the rear 02 sensor since it is original with 127k on it, have a look at the EGR condition and then go from there - hopefully it won't be another cat, but I'll post what I find in case anyone else can get some use from it. Daggone thing runs great, otherwise!

Todd.
.

Everyone,

I drive a 1998 Accord LX (4 cylinder) with 227,000+ miles on it. For several years I have had this issue. Several times the P0420 reading comes up in my emission test or when I take a reading with a code reader. And, each reading the P0420 code is the only code. I have found that if I drive 65 MPH for about 20 - 40 miles in one drive it will clear and I pass emissions immediately. I usually drive 75 - 80 MPH, and when I do the check engine light (CEL) comes back on and the code is P0420 again. For a few years now, I just slow down before my emission test until the CEL is gone, then drive another 20 miles to reset to the ready state, and get emissions checked. Just FYI for everyone. Works for me; hopefully, it will work for others also.

I know I don't know a whole ton about cars, but have you tried using a higher octane fuel? Sometimes cheaper gas will screw with the catalytics because it's "dirtier"

Hey all, came across this forum and thread by googling the p0420 code, and have read the entire thread. I have a 03 accord 2.4l 5speed with 175,000 miles on it. got the p0420 code and I replaced the cat conv. with a direct fit but not from honda. drove over 50 miles and the code came back on.

any ideas? I guess ill try cleaning out the EGR valve. who can I go to to get a really good reading telling me exactly what the code is? I live in Atlanta, GA by the way.

thanks and WHATS UP?! first post newbie here. :)

Not all CATs are interchangeable. Maybe this is the wrong CAT.
Can you tell which type of car you have: LEV, ULEV, SULEV?
Link to CAT used:
--------------put hear----------------------

Other thought, is did you replace the rear 02 sensor?
What did u used?
How is your oil and anti freeze consumption?

I used this CAT:

http://cgi.ebay.com/ebaymotors/2003-Honda-Accord-catalytic-converter-2-4L-4cyl-NEW-/320645369979?pt=Motors_Car_Truck_Parts_Accessories&hash=item4aa7f4107b

and its a LEV, and I havent replaced either o2 sensor yet. I did run a full can of cleaner through the throttle body. Ive also reset the computer and drove it 50 miles twice now. I was just trying to get an idea of IF I need to change the o2 sensor, or if I should go to honda to get a diagnostic. How much do you think honda would charge for that?

Hello,
I also have been dealing with these issues. It began with P1399 and was told that the issue was my distributor, so I replaced it last week and now I am getting the P420 code. I don't understand that because I replaced my CAT about 20k ago and all was good till recently my CEL came on all the while the car ran fine at higher RPM's; however, at idle it would idle rough and my CEL would at times blink. So I have thus far replaced: Plugs, wires, COMPLETE DISTRIBUTOR (recommended by dealer), CAT (about 1.5 year ago).
Today I ran diagnostics and came up with the P1399 as well as P420. I have been reading and the only thing I have not done is replace O2's and EGR.

Anyone have more suggestions outside of this thread? Match and lighter fluid are not an acceptable repair! I have thought about it though!

Thanks,

Clem

Hello,
I also have been dealing with these issues. It began with P1399 and was told that the issue was my distributor, so I replaced it last week and now I am getting the P420 code. I don't understand that because I replaced my CAT about 20k ago and all was good till recently my CEL came on all the while the car ran fine at higher RPM's; however, at idle it would idle rough and my CEL would at times blink. So I have thus far replaced: Plugs, wires, COMPLETE DISTRIBUTOR (recommended by dealer), CAT (about 1.5 year ago).
Today I ran diagnostics and came up with the P1399 as well as P420. I have been reading and the only thing I have not done is replace O2's and EGR.

Anyone have more suggestions outside of this thread? Match and lighter fluid are not an acceptable repair! I have thought about it though!

Thanks,

Clem
Please specify your car exactly. Model (engine and transmission), year, current mileage, any service done to the engine at how many miles and years ago(valve clearance adjustmet, plugs etc.).
Then we can try to help you.

WOW! I actually didn't post that really important stuff, duh! I have a 2000 Accord EX 2.3 5 speed, 238k. No other info other than that.

When was valve clearance adjustment done the last time? If you don't know exactly when then I would strongly recommed to do it now.

I have never done anything like that on a Honda, is it pretty much the same as on an american (chevy) v8? If so, what should the clearance be? Is there a specific order or proceedure in which to do this?
Thanks!

Okay,
I adjusted the valves back lash, they were at .22 intake and .25 exhaust. I adjusted these to .27 and .30.
Also, the very first thing I noticed is that the plugs were not very tight, by that I mean they just snug at best... wonder if that was the problem causing the P1399.
I also removed the EGR and checked it, it looked fine, no real obstructions on the ports.
Last, I scanned it and only came back with the P420 (bank 1). I'm going to drive and see what happens next, I'll keep you posted.
Thanks for info!

Clem

Okay,
38 miles later and my CEL is back. I just scanned it and the same P420 (bank 1). I am starting to think that I should replace the O2, but which?
Any ideas?

Thanks,

Clem

Hello Clem,

According to repair manual:

DTC P0420: Rear Bank Catalyst System Efficiency Below Threshold (Bank 1)
DTC P0430: Front Bank Catalyst System Efficiency Below Threshold (Bank 2)


NOTE:

•Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information.
•These catalysts are mounted to the cylinder head.
•If some of the DTCs listed below are stored at the same time as DTC P0420 and/or P0430*, troubleshoot those DTCs first, then recheck for DTC P0420 and/or P0430*. P0137, P0138, P0157, P0158, P2270, P2271, P2272*, P2273*: Secondary HO2S (Sensor 2)
P0141, P0161 *: Secondary HO2S (Sensor 2) heater

•Poor quality fuel can cause these DTCs.
•Information marked with an asterisk (*) applies to the front bank (Bank 2).
1.Turn the ignition switch ON (II).
2.Clear the DTC with the HDS.
3.Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
4.Test-drive under these conditions:
•ECT SENSOR 1 above 158°F (70°C)
•Transmission in D position
•Vehicle speed between 45 - 75 mph (72 - 120 km/h) for 5 minutes or more with cruise control set
•Maintain the vehicle speed at 55 mph (88 km/h) for 5 minutes or more with cruise control set
5.Monitor the OBD STATUS for DTC P0420 and/or P0430* in the DTCs MENU with the HDS. Does the screen indicate EXECUTING? YES - Go to step 6. NO - If the screen indicates OUT OF CONDITION, go to step 4 and recheck. If the screen indicates PASSED, intermittent failure, the system is OK at this time. If the screen indicates FAILED, go to step 8.
6.Continue test-driving until a result comes on.
7.Monitor the OBD STATUS for DTC P0420 and/or P0430* in the DTCs MENU with the HDS. Does the screen indicate FAILED? YES - Go to step 8. NO - If the screen indicates PASSED, intermittent failure, the system is OK at this time. If the screen indicates EXECUTING, keep driving until a result comes on. If the screen indicates OUT OF CONDITION, go to step 4 and recheck.
8.Turn the ignition switch OFF.
9.Replace the WU-TWC
•For DTC P0420, replace the rear WU-TWC (Bank 1).
•For DTC P0430, replace the front WU-TWC (Bank 2).
10.Turn the ignition switch ON (II).
11.Reset the PCM with the HDS.
12.Do the PCM idle learn procedure. See: Programming and Relearning
13.Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
14.Test-drive for about 10 minutes, continually changing the vehicle speed.
15.Check the DATA MONITOR CONDITION B1/B2 in the DATA LIST with the HDS. Is the temperature OK? YES - Go to step 16. NO - Go to step 13 and recheck.
16.Test-drive under these conditions:
•ECT SENSOR 1 above 158°F (70°C)
•Transmission in D position
•Vehicle speed at 55 mph (88 km/h) for 5 minutes or more with cruise control set
17.Monitor the OBD STATUS for DTC P0420 and/or P0430* in the DTCs MENU with the HDS. Does the screen Indicate EXECUTING? YES - Go to step 18. NO - Go to step 16 and recheck.
18.Continue test-driving until a result comes on.
19.Check for Temporary DTCs or DTCs with the HDS. Are any Temporary DTCs or DTCs indicated? YES - Go to the indicated DTCs troubleshooting. NO - Go to step 20.
20.Monitor the OBD STATUS for DTC P0420 and/or P0430* in the DTCs MENU with the HDS. Does the screen indicate PASSED? YES - Troubleshooting is complete. NO - If the screen indicates FAILED, check the fuel quality. If the screen indicates EXECUTING, keep driving until a result comes on. If the screen indicates OUT OF CONDITION, go to step 13 and recheck.

Sounds like you need a new catalytic converter. Here is another post about similar problem: http://www.v6performance.net/forums/1768522-post43.html

Even though mine has less than 30k on it?

Even though mine has less than 30k on it?
Yes. If there is a problem with your engine then you can kill a catalyst very quickly. That's why it is the best to do a comprehensive diagnostic on the engine.
But some vehicles are prone to burn CATs very quickly. Toyota 4Runner 2003 6-cyl is a good example (the bad EFI program is the cause for this).
If the valves were out of spec and you drove the car ignoring CEL then you burned excessive amount of fuel in your CATs and they gave up quickly.

My code is on a 2001 Civic LX 4cyl 5sp VTec ULEV

Glad I'm not the only one with this issue. I started having a power loss problem initially. When the car was cold (not during the winter, wasn't a freezing issue) it would have a complete power loss, but never stall. Pushing the clutch in and waiting a few seconds the rpms would return to normal and after starting to drive again it would sometimes continue to bog down a few more times. After five minutes or so it wouldn't happen again. During this issue the CEL never came on. Hoping it was a real bad batch of gas I ran the tank down to almost empty and filled again. The problem didn't stop at that point.

The car has sat for 2-3 months at a time for the past 2-3 years. The power loss problem seems to happen much less often now, but I got the P0420 code, after clearing it the last time it returned a P0420P almost immediately. We want to sell the car so I really don't want to just start throwing money at it without a better idea of the actual issue. I'm starting to lean toward changing the O2 sensors, starting with the one before the CAT. I was thinking that an O2 sensor could make sense and that as the exhaust heated up it began to function better. But I'm not sure if an O2 sensor could effect the power loss that much. I was of course told I needed a new Cat, but I'm going that route until I know for sure it's bad, that's 20% of the value of the car. Any additional help someone can offer would be greatly appreciated, thanks in advance.