po420 mystery

Asked by Jan 28, 2017 at 09:56 AM about the 2008 Chrysler Sebring Touring Convertible

Question type: Maintenance & Repair

I own a 2008 Chrysler Sebring Touring Convertible with a 2.7 liter engine with
64000 kilometers on it. My problem is that I am getting a p0420 code
showing up every second day or so. I know it means that the catalytic
convertor is not working up to par. Here is what I have done to try and get it
fixed. I have taken it in three times and they say the catalytic convertor is
working properly after doing all kinds of tests on it. The car gets great gas
mileage and it has never dropped from when I purchased the car. There is
lots of power. I was told that it was the upper O2 sensor, so I had that
replaced. The code came back 2 days later. They changed the lower O2
sensor. The code came back. They put it back on the scanner and could not
find anything else wrong. They said to run seafoam through it and take it on
the highway for a long run. I did that twice. The code came back. Back on the
scanner and still nothing showed up but the p0420 code. They said to try and
change the gas that I used. I did that. The code came back. They said to buy
a scanner and remove the code whenever it came back, because they
cannot find anything else wrong with it. I did that and after every trip with the
car I plug in the scanner and reset the computer. Now for the interesting part,
whenever it is colder than -10 I can drive the car for weeks without the code
showing up. As soon as it gets to -9 the code comes back. I am at a loss on
what to do now so I hope someone can tell me what to do.

2 Answers

52,595

Is it possible that you have a small exhaust leak somewhere upstream from the catalyst?

1 of 1 people found this helpful.
70,875

Other than what Bob said the only other things that can set a false catalytic converter code are a bad downstream oxygen sensor (the upstream oxygen sensor has nothing to do with this) or a bad electrical connection for the downstream oxygen sensor. The computer (PCM) should basicly see a "flat line" signal from the downstream oxygen sensor. If there's too much fluctuations in the signal a code P0420 is set and the check engine light comes on. The PCM runs diagnostics on the catalytic converter by enriching the air to fuel mixture momentarily when you're driving. This will increase emissions output from the exhaust unless the catalytic converter is working properly and "eats" the excess emissions. If the catalytic converter is NOT working properly the PCM sees the excess emissions as a spike in the signal from the downstream oxygen sensor so the diagnostic test fails. If the catalytic converter IS working properly there's no spike in the signal from the downstream oxygen sensor and the diagnostic test passes. After three failures the P0420 code sets and the check engine light comes on. The PCM will only run the diagnostics under specific conditions while your driving. The gas tank must be between 3/4 and 1/4, etc. Extreme temperatures will prevent the diagnostics test from being run. This explains why if the ambient temperature is below a certain point the check engine light will not come on for several days. Chances are if you checked, the catalyst readiness monitor would not be set. This is because the PCM has not run the catalyst efficiency test three times. The P0420 code rarely sets unless the catalytic converter is bad. Since it's the last line of defence before the exhaust hits the atmosphere it's a very reliable testing set up. If the code keeps setting, you need a catalytic converter. That's the bottom line. If your mechanic doesn't want to replace it, for whatever reason, go somewhere else. A good muffler shop can do the job. You can also save a few bucks by getting a universal converter. Just make sure it's compatible with the emissions system on your vehicle. The emissions decal under the hood will tell you that information. HTH. -Jim

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