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Discussion Starter · #1 ·
I have a 2003 Honda Accord EX Sedan w/ 2.4 L and its throwing a P0420 code after I just replaced the upstream O2 sensor. Everyone says / thinks that its a catalytic converter problem but I think that its something up with the ignition system since when I'm at a light idling its super rough, like I feel like the car is going to fall apart! I just want to make sure that if the catalytic converter is out that it is indeed out before I dump more money into it. How do you even tell if its going out or is out?!
 

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well if its clogged you can get it nice an warm (drive hard for about 15 minutes) then prop the car up and knock it a few times with a hammer. will loosen up the carbon (when its warm) and you should be good to go for a while:thumbsup:
 

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Discussion Starter · #3 ·
Oh and it has 170,xxx miles on it. I just need to know so I can get whatever parts that I need for it so it can pass inspection this upcoming month...
 

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last time you changed the oil? cleaned the maf sensor/ tb/air filter?
 

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I change the oil every 3k miles. I have never cleaned the MAF sensor, and I don't know how to lol fail. tb? The air filter about 8k miles ago I want to say.
 

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did you reset the ecu after you changed the o2 sensor and tb= throttle body
 

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Discussion Starter · #7 ·
I didn't resent the ECU either, I was driving and it just went away and I don't know how to haha. I also haven't cleaned the throttle body..
 

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Discussion Starter · #9 ·
Would the downstream sensor cause that code to get thrown? Sorry, just making sure. I'm a broke college student
 

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I change the oil every 3k miles. I have never cleaned the MAF sensor, and I don't know how to lol fail. tb? The air filter about 8k miles ago I want to say.
The MAF sensor cleaning is even easier than changing your oil ... one of the forum stickies has the link to the DIY w/ pics (I think)
 

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"Would the downstream sensor cause that code to get thrown? "

Yes. A P0420 code is related to the downstream O2 sensor, which is on the rear facing side of the catalytic convertor. When a car throws that code, it usually means that there is something wrong with the downstream sensor circuit, or the downstream sensor is detecting a similar reading to the upstream sensor (it shouldn't). In the latter case, this can either be caused by a faulty downstream sensor, and/or a catalytic convertor that is not working properly. Spark timing that is retarded too far may also cause this code to arise, as well as using leaded fuel.

However, a P0420 code set by a faulty catalytic convertor shouldn't cause serious drivability problems like you are experiencing. The downstream sensor doesn't help the car run smoothly - it's only there as part of your emissions monitoring system. I would look elsewhere if your car is running that poorly, especially if it occurred fairly suddenly. I would verify again that you are not getting codes relating to the upstream sensor, and check the connectivity.

Also, I would refrain from cleaning the throttle body, unless you are familiar with that process as well as how to do an idle relearn procedure (they are electronic). Also, be very careful with cleaning the MAF sensor - only use MAF sensor cleaner (not throttle body cleaner).

Other than that - you might check your spark plugs and wires for connectivity and wear and verify that all your injectors are working properly.

Good luck.
 

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The P0420 code is a symptom of some other issue, all the code is saying is the Sensor after the catalytic convertor is measuring a changing waveform instead of a flatline. Catalytic convertors do not "wear out" they are killed by lean or rich fuel mixtures, contamination, or physical damage. So the P0420 error is the ECU making a decision based on the readings from the O2 sensor behind the catalytic convertor.



So what you need to look at is

1. Are there any other codes present or pending in the ECU, if there are you need to look at those codes first.
2. Look at the freeze frame data when the P0420 code was set to determine what the fuel trim, speed, rpms and other parameters looked like when the code was set.
3. Physically look at the Catalytic Convertor and make sure there are no dents rips or tears in the metal around the converter, also look to make sure it is not discolored indicating overheating.


Throwing parts at this issue is going to be expensive and not fix the issue. You need to troubleshoot the error, not replace parts.


Read this article, very informitive
http://www.easterncatalytic.com/education/tech-tips/the-misleading-nature-of-the-po42o-code/
 

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