P0420 OBD-II Trouble Code

[Armor01]

Sorry if this is a bit winded but I will try to give you all the needed info to try to help me nail this down. 1998 SE 3.1
About four months ago the Check Engine Light (CEL) came on an gave me the code for the rear oxygen sensor. Replaced it and all was fine. About a month ago the CEL came on and the local parts store asked me if my gas cap was ok. I had looked at it earlier and it was cracked, so I replaced it with an aftermarket brand. A couple days later the light came on and I took it back and they replaced the cap because the recently installed one would not hold and stay tight. Came off after it was tightened. Light came on again and I got to see the code this time. It was P0420 - Catalyst System Efficiency Below Threshold (Bank 1). I was told that it could be the oxygen sensor and since the rear was replaced I installed a new one before the Catalyst. RAN GREAT. STARTED BETTER. Yup, light came back on and the same code came back and this time the gentlemen told me that I might have water in my gas. At this point I'd try almost anything, so I threw in some gas treatment and the light stayed off for a week. Reset it again. The CEL has come on while I am on the highway and it has come on in the same location after getting off the highway, twice. 5 days a week, 35 miles to work from Elizabethtown to Louisville, KY at 75 mph. No loss of noticable power or gas mileage. No engine misses or other problems. Purchase name brand gas. Light has been out for four days now. Does not come on right after a fill-up. HELP, please.

[Rockfan1815]

i get the same code on my 96 grand am with the 2.4. my car still gets 24mpg even with a lead foot. both sensors before and after were replaced. rear by previous owner and front by me. im pretty sure it just means that the catalytic converter is past its prime and cant clean the exhaust efficiently anymore. there is no power loss or anything, no stalling or surging. ive also noticed exactly what you said about the fuel additive, and it does help delay the light from returning for a few cycles.

i have my own code scanner and when i clear the code, it takes two cycles or two trips where the engine gets up to temp for the code to reappear.

[Rockfan1815]

http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php
http://www.myjdmphoenix.com/images/04_xB_Repair_Manual/05%20-%20Diagnostics/SFI%20system/catalyst%20efficiency.pdf
some info on it


my guess is your cat took a crap

[Armor01]

At a 118,000 miles, well maintained and this is what ya get. I borrowed a code reader and I plan on keeping the car so it looks like I too will get my own decoder and just keep on clearing the damn thing. You never had a loss of coolent did you?

[Igor Gavric]

Loss of coolant via exhaust (erupted head gasket, hydro-lock, etc), basically any way that coolant went through your exhaust will decrease the life of your cat... Sad but true...
Replacement of cat is in need my friend, as this could mean a failed emissions test (if your state does them).

[rixGAphx]

At a 118,000 miles, well maintained and this is what ya get.Your factory cat was guaranteed for 8 yrs/80k miles.

You got your money's worth from the original cat, time to buy a replacement.
No big deal.
Why is re-setting a code (even if it is *just* the cat) better than correcting the actual problem?

Have you scanned the code recently?
You can be getting an occasional P0402, then an occassional P0xxx; different problems that you're mistakely lumping together.

And by 'well-maintained', we assume your plugs and wires have less than about 60k miles on them?

Hope this helps,
-Rick

[Armor01]

Plugs and wires are less 25,000 miles old, oil changed at 3,000 miles and no more. I have not had a coolant problem but I did see where it would kill the CAT if someone had that problem. As of 5 Dec the light has not come back on....yet. If it returns then it looks like SANTA will be bringing me a new CAT. Keep you all posted.