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  #1  
Old 04-05-2005, 08:44 PM
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unclebuck48 unclebuck48 is offline
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Angry Engine code P0420

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 . 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.
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Old 04-05-2005, 11:13 PM
tafettawhite tafettawhite is offline
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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
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Old 04-06-2005, 10:11 AM
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what year/model are we talking about here????
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Old 04-06-2005, 11:57 AM
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tymchenkop

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.
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Old 04-06-2005, 02:34 PM
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If it's a '98-'02 there's a high likelihood of it actually being a cat. That's the code that says it!
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Old 04-06-2005, 02:43 PM
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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 doesnt 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 500F.
If the outlet temperature is more than 100F 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 100F hotter
than the inlet temperature, replace the converter.
Attached Images
File Type: pdf 98-99 V6 CAT 03-073.pdf (80.6 KB, 3912 views)
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  #7  
Old 04-07-2005, 05:18 PM
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Ahh geez, I guess I never said It's a 1998 Accord LX Coupe 4cyl.
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Old 05-30-2005, 05:28 PM
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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.
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Old 05-30-2005, 05:32 PM
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Welcome pa0101!!
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the latest in a long line of Hondas:

His: '06 Ridgeline RTL with Nav Steel Blue, rear view & cargo cameras, iVES, hitch, INfinity speakers, running boards, alternate front grille, garnish, roof rack, now with 18" Ridgeline rims

little '98 Civic sedan, commute beater car. Named Mater.
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  #10  
Old 05-30-2005, 07:04 PM
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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
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Old 05-31-2005, 03:54 AM
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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?
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Old 08-08-2005, 03:45 AM
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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.
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Old 03-07-2006, 07:46 PM
my_vtec my_vtec is offline
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Quote:
Originally Posted by unclebuck48
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 . 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.
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.
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  #14  
Old 04-20-2006, 02:40 PM
bleedinred bleedinred is offline
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Quote:
Originally Posted by my_vtec
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.
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Old 06-07-2006, 09:07 PM
my_vtec my_vtec is offline
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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
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