P0420 CEL 2001 Monte Carlo SS
 

I just had my CEL come on last Sunday when I was down in Des Moines. I went to O'Reilly's and they gave me the scanner I went out and it came up as P0420 Catalyst Efficiency Below Threshold. I went home and unhooked the positive terminal for about a half hour and it cleared. It then came back on Friday afternoon so I had it scanned at Advanced Auto and the same thing came up. Today it started running a little hard when I am in the gas. It still get's up and goes no problem but it has a vibration throughout the car that wasn't there before. I can see this progressively getting worse but am pretty sure its the catalytic converter. If so I have read on here that it may be under GM warranty if it falls within a certain VIN #? I am going to the GM dealership here in Cedar Falls tomorrow to have it checked. I have no moisture dripping out of the rear outlets but my MPG is steady around 30 on the highway. (just got off a 3 1/2 hour trip from Sioux City) I am looking for some input on the matter. Thanks guys!

Cat's probably going bad...

Would replacing it with a 3" bolt in from ZZP be a viable option if the dealership doesn't fix it for free? I want to be able to do it myself if they wont plus I can get a little performance for the same price as one from the auto stores.

That downpipe will probably still throw the CEL.

If it is your cat - and the dealer won't replace it - another option to check into is the Magnaflow downpipe w/cat. I replaced mine about 3 months ago and (so far) I haven't got a P0420 code. The cat flows a little better than stock and it will eliminate the u-bend that the stock downpipe has.

Update:

Went to Rydell Chevrolet here in Waterloo and he showed me the printout. The cat was replaced at 63,000 miles once under warranty but the special policy closed for the car on 9/21/2011. I missed it by 2 months so he said they would meet me 50/50 at $346.31. Problem is I still don't know for sure it's the cat, so I have a appt. at 1:30 with Midas to have them tell me exactly what the problem is, catalyst, o2, etc. Depending on the cost I might just do it and have it over with or if I decide to do it myself buy the parts and do it the next time I am home from college.

For the same price of $350 you can get the downpipe with cat + a canned tune that will erase the P0420 code ;)

:iagree::thumbsup:

Midas confirmed it was the in fact the catalytic converter and not the O2 sensors. They say the brick is broken as they can hear it rattling inside the casing. It's not plugged yet but make break up more and then become plugged. I have a few options: order a cat from ZZP and a PCM for 350 or so or buy a magnaflow bolt in. I just want it to work and not have a damn code!