Thread: P0420 code

I have a 2002 Grand Prix with a 3.1L engine. Recently the check engine light came on. I went to Auto Zone and they pulled the codes. It came up with a P0420 code. The guy at Auto Zone said that the problem was probably an O2 sensor.

The car smells of gas when sitting at idle but it doesn't run bad. I haven't noticed a lack of power but the gas milage has dropped quite a bit. Will a bad O2 sensor cause it to run very rich?

I read through all the posts (all 57 pages of them) but I couldn't find any with similar problems. Does anyone have any other idea's or should I replace the O2 sensor and go from there?

Any help would be appriciated.

You're catalitic converter is going out and more than likley getting clogged up.


You can do three things.


1. Ignore it.

2. Replace the cat

3. Replace the cat with an off road downpipe (bolt in or weld in) wich will give you a little performance. That is if you don't have emmissions. You can get one from ZZPerformance and would more than likley be just as much as a new cat.

I thought of the cat, but the car only has a little over 60,000 miles on it. That's why I was thinking that the problem was somewhere else and not the cat.

I had the same code on my 2001 3.8 and replacing the O2 sensor cleared the ses light. Not sure about the impact on mileage yet as I haven't ran through a tank of gas yet.

Thanks for the answers. From what I've heard a bad O2 sensor can cause the engine to run rich. This would explain the gas smell and the bad milage. I think I'll try an O2 sensor and see if that solves the probem. I'll let you know if it solves it.

I replaced the O2 sensor yesterday and that seems to have been the problem. No more gas smell and the code hasn't come back. I'll post again if it turns out that wasn't the problem.

If that 02 sensor was bad you more than likley would have gotten an 02 sensor code. Not saying it can't happen but I have never seen or heard about that code associated with a bad cat o2 sensor.



That o2 sensor does not contral any fueling. Al it's there for is to monitor the cat. That is it.


In the PCM there is a table called CAT over temp protection. When you reach a certain engine temp via the ECT then it commands a richer AFR ratio. All the way down to 9.12 wich is extreamly rich.


This may have been what you were smelling.

Are those sensors easy to replace on your own?. I paid a damn shop 300 to change one and now they said oh we think the other one is bad too.