Code P0420

[kallenbach_05]

i have a 99 grand am 2.4L, i get this code about once a week and i just keep resetting the light. is the cat going bad or is it already bad? or what else would it be? have around 115,000 miles on it

[xeroinfinity]

It could be, you'll need it properly tested to find out.
Not a cheap test either..

Good Luck

[J-Ri]

It it was me, I'd just replace both O2 sensors. They'll run ya about the same as having it looked at. You probably need the upstream sensor if you havn't replaced it already, they usually go about 100K. If the code comes back, most likely a bad cat. If the cat is bad, there is most likely a combustion problem that took it out.

[Headnsouth]

http://www.aa1car.com/library/p0420_dtc.htm

I've got the same DTC on my 98 Transport w/172K miles. I'm running some auto-rx thru the crankcase right now and I'm hoping it'll clear. I did have 2 other DTC before I poured in the bottle of ARX. So far 2 have cleared. Both were misfire codes.
www.auto-rx.com

[xeroinfinity]

You can test the O2 sensors to varify they are faulty.

Oxygen Sensor- 99+ Grand am - Testing and replacement

But as stated above, at this point in mileage it wouldn't hurt to change them both, though I'd hate to spend $200 for new O2 sensors and still have to buy a converter.

Good Luck

[kallenbach_05]

both o2 have been changed. i was kinda figuring that the cat would be bad. i had a misfire problem that took awhile to get figured out so thats pry what hurt the cat