Code P0420- what's it mean?
 

Never had a CEL or other warning lights but my car had this code stored. What does it stand for and what are the implications?

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Just erase and see if it is coming back....

I get them periodically. The cat isn't cleaning the exhaust enough. This is informational only - no effect on performance. Your tune suppresses the CEL for this code, which is why it is just stored.

Jon

Nothing to worry about. Common with aftermarket exhausts especially ones with less than 200 cell cats. Just reset it. Probably wont return for a long while. If it comes back repetitively and often you may need to talk to exhaust manufacturer about some new cats.

I accidentally put 6 gallons of leaded 110 race fuel in the tank at the track last year. Hope this is not now coming back to haunt me. How will I know if the CAT has gone bad? Could this have anything to do with the intermittant limp mode I am experiencing? Just had the alternator and battery as well as the MAF changed, and I still have intermittant limp mode. Also had the system pressure checked, no leaks.

Yes, could be leaded fuel but too early to conclude that. Just reset and wait.

As far as limp mode, inadequate cats do not put car in limp mode. In my experience, intermittent limp mode is usually old plugs or coils,

Plugs have 4000 track miles on them, do you think they are finished? Coil packs are 1 year old.

IMHO, that is most likely the problem. 4K on the track is a lot more than 4K on the street. If you have any doubts you could pull one of the more accessible ones and have a look...

Just a thought, but doesn't leaded gas clog up cats? Could his problem be due to lack of exhaust flow? I can't imagine 1 tank doing this kind of damage, but wanted to throw it out there just in case.

Wasn't even a tank full, just 6 gallons.

Didnt know this before checking but, lead will immediately coat catalyst and reduce efficiency. Two tanks of leaded fuel can render cat useless. The cat does not have to be clogged to be useless. If enough lead is deposited then it will not clean exhaust well enough to avoid CEL. Many performance cats barely clean well enough to avoid CEL anyway so any lead coating can tip them over the edge...
Some info here...
http://www.ehow.com/catalytic-converter-laws/

Code 420...Time to spark one up.