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Discussion Starter · #1 ·
My Accord is the 2006 PZEV version with 92,000 miles. A few days ago I got the P0420 Check Engine Light which stands for Catalyst System Efficiency Below Threshold (Bank 1).

Plugged in an OBDII tool and found that my Short Term Fuel Trim is at around 20% when idling and drops to 0 when I increase RPM to 3000. Long term fuel trim can go up to the high teens but drops to about 4% after a 20 minute driving trip.

I brought the car to Honda because I thought it might be the the catalytic converter which is covered by the warrantee. The technician at Honda updated the ECU software and removed the trouble code. He told me that everything is good to go now and it shouldn't give me any more trouble. I tried to talk to him about the fuel trim thing but he didn't think it was an issue at all.

Is it abnormal for the fuel trim to go up to 20% when idling? Is it something I should look into or just keep driving the car like the Honda tech told me?
 

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A high fuel trim while idling might point to a vacuum leak. Vacuum leaks are more prominent while idling and they decrease when you rev the engine. Not an expert. I just keep reading material over the internet. But if the Tech says you are good to go, then I don't think you need to worry.
 

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Discussion Starter · #3 ·
Vacuum leak is what I thought as well after some internet searching. I'm going to get a can of carburetor cleaner and try to find where the problem is.
 

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I am no expert and am only going on my own fuel trim readings, and to boot, I am an owner of a '98 Accord. But...

My short term fuel is never is high as "20%". Do you always get similar readings or was it just a spike? Is it still that much after the dealer removed the trouble code? My short term fuel trim generally bounces between -5.0 and +5.0 at start up, and after warming up settles in it bounces around -2.3 to +2.3, maybe more at stop lights. The more important long term fuel trim, however, on my car pretty much always settles down to 0.0 or extremely close to it after a short time driving. I'm not sure, but I think this is optimum. Your 4% long term fuel trim and your 20% short term fuel trim seem a little high to me but maybe its the difference in our cars or maybe it's just within specs??? Sorry I don't know the answer.

Have you experienced a sudden decrease in gas mileage?

I'm not sure on your model, but you probably have several 02 sensors:

http://www.bernardiparts.com/SearchOptionsV7.aspx?Search=MODEL&ModelID=honda-accord-parts

If it's not a vacuum leak, could your pre-cat or your air-fuel ratio sensor (#16) be faulty? Hopefully it's not your cat going out at 92,000 miles.

If you already haven't, take a peek at this for your P0420 and warranty issues:

http://www.tegger.com/hondafaq/p0420/index.html

Good luck.
 

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Sorry, the bernardi parts link didn't work right. Enter your car info and go to "electrical, exhaust, heater, fuel" category and then to "exhaust pipe" to see where the sensors are.
 

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Even short term fuel trim going to 20% is not good, as that is the maximum change short term fuel trim can make. At that point it would move the long term fuel trim positive to compensate. Total fuel trim is short term PLUS long term.

I suspect you have a vacuum leak.

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Discussion Starter · #7 ·
I tried finding a vaccum leak with carburetor cleaner spray but couldn't find it. Maybe I just haven't hit the right place? What are some common places that a vacuum leak might develop?

I also let the car idle to see if a high Long Term Fuel Trim would turn on the CEL but it never did. LTFT topped out at 18.6%, which isn't high enough.
 

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Discussion Starter · #9 ·
Thanks for your help so far. Another strange but possibly related thing happened today. I got the "CLOSE GAS CAP" message on the instrument panel which corresponds with P0456 - Evaporative Emission System Leak Detected (very small leak).

My gas cap was definitely closed. The interesting thing is if I open the gas cap the Short Term Fuel trim drops to nearly 0%.
 

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Sorry, I am still not familiar with your Honda model but we are both getting the same codes..

...I had the same "Evaporative Emission System Leak Detected" code (though I thought mine read, "P1456". Maybe I was wrong on the numerical code but the description was the same. I had changed the gas cap a year ago but the CEL kept coming back but at a slower rate. I suspect I was losing fuel/mileage and that you are too. In my case I had an outright drip-drip-drip puddle forming fuel leak. Kinda scary since there should be no fuel in the EVAP system as I understand it.

I had the EVAP bypass valve and solenoid valve replaced ($180 parts+80 labor) and no CEL in over 800 miles and two fuel-ups so far, knock on wood. Jury's still out, but I will update my other thread in a few weeks when I am totally satisfied. Also, I can now hear the outright "swoosh" from the pressure retention when opening the gas cap to refuel, something I didn't hear before the work was done on the car. I hope that's a good sign that all is working well again.

I had developed a fuel leak that, thank goodness, forced me to identify and change the parts. I'm not saying you have the same problem, but just have a look at the thread below. I was away from home otherwise I may have tackled replacing the parts myself. Dunno. I'm weary so far of working on fuel-related problems myself. But it looked easy enough when I saw the removed parts--a couple of hoses attached, mounting bolts, and an electrical connector on the solenoid.

Look under your car for a leak with a cold engine and then again with a very hot engine--try at night with a flashlight, too--for fuel vapors (or obvious leaks). I was shocked to see both the leak and the steaming vapors (hot engine) coming off the EVAP system. I don't think even seeing the vapors is normal. It was probably the fuel leak and/or the fuel evaporating onto the hot parts.

Since the repair everything appears bone dry. No leaks, no vapors and no codes so far. Just a pleasing $260 "swoosh" sound. :)

http://www.driveaccord.net/forums/showthread.php?t=163682

P.S. The replaced parts are NOT shown in the above thread, just where it was leaking from/onto. I think they were contained in part in the smaller, fuel-soaked plastic box shown. Again, our cars are years apart so I'm not sure what your system looks like.
 

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The evap system can be your source of a leak as all that system does is collect the vapors from the gas tank and store them until they can be burned in the engine via vacuum and valves that open and close. So your real issue most likely exists in the evap system.

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@RMN: Which state you live in?
 

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Are you over-filling your gas tank? Just a thought.
 

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Almost all of the EVAP components are back by the Gas tank, there should be a Vacuum hose routing diagram showing where the Vacuum line leading to the EVAP system comes into the engine. For a test I would try to pinch off that vacuum line and watch the fuel trim. As far as the P0456 code, I assume you did verify that was the code in the PCM. That code has is not set until the error happens on 2 drive cycles, on the first error it is stored as a pending code. Basically the computer pressurizes the system and the pressure sensor measures if there is a drop in pressure, if that happens in 2 drive cycles the MIL is lit and freeze frame data stored. The light will go off after 3 successful test. If the Gas cap and filler neck are ok then time to look at the hoses and EVAP canister for holes and/or cracks and test the various purge and other valves in the system. For that a manual would be nice.
 

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P0420 = new O2 sensors. Your car is asking for you to replace them and you won't.
 

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Late to this but I'm seem to be in a very similar situation. Car has been running absolutely fine but the P0420 comes back every 500 miles roughly. I clear it because the light bothers me lol. Did you ever have this issue fixed? If yes, what was the culprit?
 

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Here is the bottom line, the sensor after the catalytic converter is sending waveforms that indicates the catalytic converter has fallen below it's original efficiency. It does not take much of drop to set the code.

There is no "magic bullet" that will fix this issue. Most likely it is a bad catalytic converter, but it is possible that the O2 or AF sensors are worn/bad and replacing them will fix the issue.

Catalytic converters do not wear out, but they can be destroyed by oil burning, coolant burning, misfires, bad fuel ratio's, and physical damage. In general if you replace a catalytic converter without fixing the underlying issue the p0420 will return.

If you live in a place where you have to have CARB compliant replacement Catalytic Converters, currently that is California, Colorado, New York, and Maine. Replacement Catalytic Converters are expensive. In the rest of the country you can get good quality replacement Catalytic Converters for about the cost of the front AF sensor.

Each situation is different and it requires you troubleshoot the issue, not throw parts at it. Good luck.

Sent from my Pixel 5 using Tapatalk
 

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Here is the bottom line, the sensor after the catalytic converter is sending waveforms that indicates the catalytic converter has fallen below it's original efficiency. It does not take much of drop to set the code.

There is no "magic bullet" that will fix this issue. Most likely it is a bad catalytic converter, but it is possible that the O2 or AF sensors are worn/bad and replacing them will fix the issue.

Catalytic converters do not wear out, but they can be destroyed by oil burning, coolant burning, misfires, bad fuel ratio's, and physical damage. In general if you replace a catalytic converter without fixing the underlying issue the p0420 will return.

If you live in a place where you have to have CARB compliant replacement Catalytic Converters, currently that is California, Colorado, New York, and Maine. Replacement Catalytic Converters are expensive. In the rest of the country you can get good quality replacement Catalytic Converters for about the cost of the front AF sensor.

Each situation is different and it requires you troubleshoot the issue, not throw parts at it. Good luck.

Sent from my Pixel 5 using Tapatalk
The greatest killer of catalytic converters is if your engine is burning oil.
I avoid Extended Performance oils, Semis and Blends. These are known to vaporize and burn easier.
 

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Late to this but I'm seem to be in a very similar situation. Car has been running absolutely fine but the P0420 comes back every 500 miles roughly. I clear it because the light bothers me lol. Did you ever have this issue fixed? If yes, what was the culprit?
Start with O2 sensors first. They're the cheapest and the easiest to replace. I have made this code go away on a half dozen cars by replacing the O2 sensors.

My daughter's Prius had this code on for a few years apparently. I replaced the sensors last summer. The code came back twice. Instead of replacing the catalytic converter, I put in a bottle of Cataclean. Stuff is $25 at Walmart. Code came on again so I put in another bottle and put 300 miles on the car last weekend. I'm gonna put on another 300 miles this weekend. My theory is this:

1. Running with bad O2 sensors dirtied up the cat and it needs cleaning.
2. Running it long and hard on the highway with Cataclean will help clean it. (I have no proof whatsoever)
3. If not, I'll take the cat out this summer and try to actually clean it. There are youtubes where guys clean it with soap and water. I'll try carb cleaner first. I might buy a gallon of the stuff and just fill the exhaust pipe with it. Then maybe some soap and water. Then blow air through it.

If I'm lucky, the two bottles of Cataclean and lots of highway driving will clean it for me. We'll see.
 
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