P0420 code on just reapired exhaust system

Hi all,

I have a 2001 Chevy Prizm that I just had the front pipe replaced on. It was not accelerating well and making noise. New front pipe and it feels like a new car.

The check engine light came on. i cleared the code, drove it for a day or so, and the light is back on.

My exhaust mechanic says we don’t really need to worry about the cat con hurting our engine unless we see a decrease in performance.

Are there any mechanics out there who would say that is true. The car has 160,000 miles and we want to see it go another 100,000 or so, and not do anything to damage the engine.


I think you may need a new mechanic, but you haven’t said enough for people to know how to advise you.

First, presumably the front pipe was all clogged up? If so, why? You need to figure that out.

Was this code present before the front pipe was replaced?

In regards to the P0420 code, what has the mechanic done to check things out? It doesn’t necessarily mean that you need a new cat. All the P0420 code indicates is that the O2 sensor in front of the cat and the one behind the cat read too similarly. Normally this would be that the downstream/rear sensor signal looks too much like a signal that should/is coming from the upstream/front signal. If the sensors are working well and everything else is in order then this would be because the cat isn’t doing its job.

It seems likely to me to first assume that something didn’t go correctly with the pipe installation where one or both of the O2 sensors and/or their wiring were damaged. But it also seems likely that you might have some other problem - for instance, if you have a bad coolant temp sensor your car might run too rich. Over time this kind of thing can contribute to clogging an exhaust and will show up in O2 sensor readings.

First, thanks for the reply. Second…ugh. Ok. Now to answer your questions:

  1. I don’t know if the front pipe was clogged. We had an exhaust leak from the front. They changed a gasket, the car accelerated great, and after 1.5 hours of driving the exhaust leak was back. I took it back to them and they said the front pipe had to be replaced. Taking them at their word that they replaced it, the noise is now gone and has been gone for over a week.

  2. The code was not present before the replacement. The code came on the day I picked the car up from the front pipe installation.

Giving them the benefit of the doubt, is it possible that a big exhaust leak, which then gets fixed, would cause the code to be thrown? Or, is it likely that the O2 sensors, after 160k miles, need to be replaced and it is conincedence.

I suppose the last option of they messed something up while installing is the simplest answer, but I would like to know what you think of those other two scenarios.

Thanks again for your great and detailed answer.

Oddly enough, it’s possible that the big hole was preventing the check engine light from illuminating. If the hole was between the upstream oxygen sensor and the downstream oxygen sensor it could have been allowing air to be drawn in and the sensors would have shown a larger change in oxygen levels than the cat converter is responsible for.

You don’t want that light on incessantly. It’s your early warning system for countless other problems and you do not want it effectively disabled. Your mechanic shold not have made this recommendation. You’ll not get another 100,000 miles out of the car if you follow this guy’s recommendations.

A new cat would not be an unusual need for a Prism with 160,000 miles. See another mechanic, have him verify that you need the cat converter, and have him replace it.

Hello. Have you serviced the front and post Ox sensors on you car ? Can you give us the readings on fuel trims and sensor ? You have a Toyota Corolla with Chevy marks, so expect to pay high price for the parts and you will have to be careful with what you replace so you will not break the bank. Please see if you can post the results here again.
Thank you.