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I have a 2003 Honda Accord EX 4DR 4 cylinder with over 170,000 miles on it. A week ago the check engine light came on. The car still ran like normal. I checked the code over the weekend with my scanner and the code was P0420 - catalyst below efficiency. Well I figured that I could just reset the code and see if it came on again. I have been driving it for a week and the CEL has not came one again. However, ever since I reset the code once the car gets up to normal operating temperature when ever I stop at a stop light/sign it feels like it is about to stall out. The idle is lower than normal around 400-500rpms. Also, when I put it in park or neutral the RPMs drop down to 400-500RPMs then after about 15-20 seconds it will correct itself and idle up to around 700-800RPMs which is normal from what I remember. I am not sure what it is. I was first thinking it was my O2 sensor in the catalytic converter? Could I just unplug it to see if it acts better then replace it if so? I know the code usually means to replace the catalytic converter but I didn't have any symptoms until I reset the code. I was also wondering if it could be the IAC valve?
 

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Your are right about the converter efficiecy O2 sensor and code have no engine control functions. Your low idle problem is a totally separate issue and just a coincidence it started when you cleared the code. I would recommend cleaning the throttle body first, if that doesnt fix it then clean the idle control.
 

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nehonda thanks for the reply.

I will try cleaning the throttle body and idle control valve and if that does not fix it then I will just replace the idle control valve.
 

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Did you do an idle learning procedure after resetting ECU?
Next is procedure for my car (I hope is the same for yours)

PCM Idle Learn Procedure

1.Make sure all electrical items (A/C, audio, lights, etc.) are off.
2.Reset the PCM with the HDS.
3.Turn the ignition switch ON (II), and wait 2 seconds .
4.Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, or until the engine coolant temperature reaches 194°F (90°C) .
5.Let the engine idle for about 5 minutes with the throttle fully closed. NOTE: If the radiator fan comes on, do not include its running time in the 5 minutes .
 

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The rear monitor sensor can have an effect on fuel trims. The PCM can drive the mixture to make the cat work as it wants. (seeing it via the rear sensor) When it can't, the code is set. Some cars, but not Hondas for the most part, will severely skew the mixture and run like garbage. Many Chrysler products come to mind.

Your idle issue is probably nothing more than the learn.
 

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drozzin was right on this.

Here is what I did. I clean out my throttle body and that did not help. I was going to take the IAC vavle off but it was a pain to get to, so I decided to do the Idle Learning Procedure. The ILP fixed it. I did not hold my RPMs to 3000 because that seems a little high. I just did 2000 RPMS. I found a service bulletin (02-051 which is at the bottom of the page) for my car that had the Idle Learning Procedure in it and it did not state anything about holding the throttle down. I assume people do it just to make sure it is up to running temperature. Once it was at running temp I let it idle for about 6 minutes just to make sure. I drove it around afterward and everything was fine.

Thanks nehone, drozzin, and Fredsvt for the quick responses!!!!:)


Do the Idle Learn Procedure
To avoid an erratic idle, do the idle learn procedure
after you install the Backup fuse. Idle learn must also
be done after updating or replacing the ECM/PCM.
1. Make sure all electrical items (A/C, radio, rear
window defogger, lights, etc.) are off.
2. Start the engine, and let it reach operating
temperature (the cooling fans cycle twice).
3. Let the engine idle for 5 minutes with the throttle
closed.
 

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Same issue

I have a 2003 Accord EX 2.4 5 speed with 193,000 on it. Yesterday, I cleared the P0420 and immediately had a rough idle. I had the CEL on for about four days prior, filled car with gas, ran some Lucas fuel system cleaner through, tightened the gas cap on, etc. I bought gas at noon on Friday on the way home from work. Got home, cleared the code, and at 3pm went to leave and the car was idling rough and at around 400-500rpm. This morning, I tried some of the procedures listed below in regards to the idle learn, but did not make a difference this morning. I drove around and the car runs perfect but as soon as you pull into a parking lot or come to a stop the idle drops down as if the car is going to stall. If you give it a little gas the idle bounces back up to around 800rpm.

I also noticed that this morning when I started the car it ran perfectly while warming up. Rather it was running at around 1500rpm for a few minutes and then as it warmed up settle down to about 800rpm as it always does. As soon as I gave it gas to back up thats when the idle dropped down to 500rpm.

I'm totally lost here. Any suggestions?

I had this code in 2009 at 102,000 and replaced the sensor myself. About six months later the 2nd one went and I think my local shop replaced that one for me.
 

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fixed for now

Changed air filter and cleaned throttle body. Slapped everything back together and the car idles normally again. I'm wondering if this caused the P0420 code to come on?? Part of me thinks the code will come back because previously it was the O2 sensor when this happened.
 

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As you may know the P0420 code is just for the converter efficiency. It has nothing to do with the engine idle or any other engine operation. As the converter efficiency drops with higher mileage, the light will start coming on more often and eventually just stay on. On rare occasions the O2 sensor in the converter can be the cause, but most times the converter needs to be replaced. If you search the internet for P0420 you can find other "fixes" also.

If your low idle speed problem returns, you may need to clean or replace the IAC on the side of the throttle body. Just cleaning the throttle body by itself will not clean the IAC.
 

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Read these 2 articles on p0420 errors,

http://www.easterncatalytic.com/education/tech-tips/pesky-p0420-codes/

http://www.easterncatalytic.com/education/tech-tips/the-misleading-nature-of-the-po42o-code/


The idle learn procedure must be done so the ECM/PCM can learn the engine idle characteristics.
Do the idle learn procedure whenever you do any of these actions:
Replace the ECM/PCM
Reset the ECM/PCM
Update the ECM/PCM
Replace or clean the throttle body
Disassemble the engine or the transmission
NOTE: Erasing DTCs with the HDS does not require you to do the idle learn procedure.

If you have the HDS you can erase codes without having to do an idle relearn!
 

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Looks like its the CAT Converter...

Thanks for the replies and the links to the info.

The car is running great after changing the air filter. No further issues with a rough idle. First time that ever happened in 194k miles.

So, the CEL came back on. I took it to my local shop for diags and they said definitely needs a cat. I'm probably just going to replace it myself. The mechanic I know told me don't buy one of the cheaper after market ones. He said to stick with a Honda OEM. He told me they quit buying the cheaper ones because the failure rate was so high.

Any thoughts on that? Thanks in advance for your help.
 

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The price difference between an aftermarket and Honda OEM is significant about $300. I would go aftermarket stay with a name brand that has a 1 year warranty. As I have said before catalytic convertors do not wear out, they are killed by bad air/fuel mixture, contamination, or physical damage. In general you should look into what made this converter go bad and make sure the underlying issues are fixed or this one will fail as well. Could be silicone poisoning or antifreeze or lead. It could be misfires or bad plugs and wires causing the air fuel mixture to run either lean/rich, or a vacuum leak. Take a look at the overall condition of the engine and make sure it checks out. Oil usage, compression check, fuel pressure check, EGR passages clean, good PCV, no exhaust leaks. Or the new convertor may not last very long.

Sent from AutoGuide.com Free App
 
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