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Check engine light - P0420 code
February 2, 2009
I have a 98 Corolla. About a month back the check engine light came on. I took it AutoZone and they pulled out P0420 code. I took it to an exhaust place where they said the Catalytic converter needed to be replaced. I got it done and paid $190 for it. The light came on again. I believe it would be about 50 miles or so after which the light would come on again if the problem is still there. Had it diagnosed at Autozone and Advance Auto Parts to make sure it was the same code. It was the same P0420 code. Took it back to the same shop and after re-diagnosis he told me both the main and the rear oxygen sensor are
faulty. He is charging $420 for it. Any advice on what i should do? Is that a reasonable price? How long can i push with out getting it fixed(i know i needs to be fixed for emissions, but there is still time)?I have heard the computer spits out a different code if the main O2 sensor is bad? I am confused...Need help...

February 2, 2009#1
P0420 code is Catalyst System Efficiency Below Threshold. I hope the mechanic told you what this actually means. It doesnt necessarily mean that the catalytic converter is "bad" - it just means the engine computer determined that the voltage reading from the rear (downstream) O2 sensor is too similar to the front (upstream) O2 sensor. It could mean a bad cat, or it could indicate a bad rear O2 sensor.

I'd say that 99% of the shops out there will blame the catalytic converter but in probably 70-80% of those cases, the catalytic converter is actually OK. Most commonly, it is the rear O2 sensor that is bad or you have an exhaust leak in front of the catalytic converter. in the case of the 8th generation Corolla - it is most likely a sensor glitch (rear O2 sensor or ECT sensor) or confused ECM.

Some more information would help - miles on the car and if it is one of the oil burners (tends to consume a considerable amount of oil between oil changes). If the car has more than 100,000 miles and uses more than a quart of oil every 1000 miles - then you probably have either a bad rear O2 sensor or bad catalytic converter. Possible that the front O2 sensor is bad - but that will flag a different OBD code.

February 2, 2009#2
To answer the oxygen sensor question, though, the sensors are usually around $30-$50 and take around one hour to replace for a beginner with a good manual.

February 2, 2009#3
PS> I agree regarding the sensors. Those should have been replaced first.

February 5, 2009#4
Thank you all for your replies. Now I am more inclined towards a bad o2 sensor. I will also check for exhaust leak.
The car has 105,700 miles. I bought it from guy in October, but i have driven only about 1000 miles so havent really noticed a drop in oil level.
I am planning to change it myself. Can anybody tell me where i can get a service manual?

Thanks again.

February 5, 2009#5
You can pickup a Haynes or Chiltons manual for around $15-$20 at an autoparts retailer. That should get you through about 80%-90% of the DIY sort of things you can do for your car. For more detailed information - you can pick up a Toyota Factory Service manual. Given the car is 2 generations oil - the hardcopy and software variants would be considerably cheaper (retail when new was over $160) - if you can still find some.

February 14, 2009#6
While i went to look at the rear oxygen sensor, i noticed there was a small hole right in front of it. When the catalytic converter was worked on they did not weld a very small portion and you could clearly feel the exhaust. That has been now been fixed, the light has been reset. I am waiting to see if that could have been the problem. If not, i will start with the rear oxygen sensor.

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