P0420 code
 

I just bought a 2002 Alero GL2 with 170K on it. Original exhaust system. Car runs great, drives great. $1500.00. Driving last night the SES light came on, and the code is P0420. With the car running great and not burning oil, I'm thinking cat converter or O2 sensors. Anyone got any experience with this? Should I reset it and see if it goes away? Just put new plugs in over the weekend, but that's the only change to the drive train so far. Thanks all. Great forum! Buffalo (in Cleveland actually).

I would reset it and see if it returns. If it does, a new O2 sensor is only about $75 at any car parts place. You will probably need an O2 sensor socket to pull and reinstall it, but most places will loan you one.
If it was the cat you should see a noticeable loss of power, especially at higher RPMs.

Thank you Iflylow! I thought I'd ask some of the experts around here. I love the Internet!

How do I reset it again?

I've already got the O2 sensors in my cart at Rock Auto just in case.

Thanks!

DTC P0420 Catalyst System Low Efficiency Bank

1. Malfunctioning oxygen sensors will set a sensor related diagnostic trouble code. Do not replace the oxygen sensor unless diagnosis following the appropriate document in SI indicates that the oxygen sensor should be replaced. For example, the oxygen sensor should not be replaced if diagnostic trouble codes P0420 or P0430 for catalyst efficiency are present alone.