Haaalp! Persistent P0420 Code :(

**Basher** · 07-19-2010

'05 GP Base, 3800 series 3 N/A, no options. Seriously. Not even anti-lock brakes.

P0420 - Catalyst efficiency below threshold, bank 1.

Brand new cat.

Upstream/Downstream o2's

cleaned out the top end (sea foam) with the old cat installed...

...

What am I missing? This is driving me nuts, and $500 invested...well, hell, that could've been performance mods. Instead of that, I've been plagued by this code since I got the car. Before I go nuts and start replacing EVERYTHING....I have to come here.

Wow me with your knowledge, GPforums. I'll love you forever if you can help me get this fixed!

**nascartech** · 07-20-2010

get a tuned pcm withe the 420 code deleted my car does the same thing after i changed the cat cause i have a high flow cat

**Bio248** · 07-20-2010

the only way for it to be doing that with all the new stuff you said you replaced is if the car is running mega ass rich so that the cat doesnt even have a chance to catch all the junk. either that or you bought a DOA o2 sensor.

**TheOtherNick** · 07-20-2010

Also some aftermarket cats, not even highflow ones can still set the code if not installed correctly or are cheapos.

**SlowNA06** · 07-21-2010

I'm gonna just throw this out there because I'm out of my mind:

Unplug the positive from the battery overnight.

**Biohazed** · 07-21-2010

I get the same thing on my car started last week just installed a ZZP Downpipe with cat and a New o2...

**Basher** · 07-21-2010

Thanks for all the suggestions, guys. If the batt disconnect doesn't work (permanently) then I'm going to get a PCM...if I can afford one. The price of the computer doesn't get me it's the damn core charge!

**Deezul_AwT** · 07-22-2010

If you get a new CAT, it's gonna throw the code. Did it on my Imp and Grand Prix. Try to find someone local with a tuner who can disable the code, otherwise you're need to get a tuned PCM.

Battery disconnect will work for 3 restarts; the code will return. And a code reader that allows a code delete will just work for 3 restarts as well.

**Basher** · 07-23-2010

Originally Posted by Deezul_AwT

If you get a new CAT, it's gonna throw the code. Did it on my Imp and Grand Prix. Try to find someone local with a tuner who can disable the code, otherwise you're need to get a tuned PCM.

Battery disconnect will work for 3 restarts; the code will return. And a code reader that allows a code delete will just work for 3 restarts as well.

Thanks for the info, bro. Yeah, I'm probably going to hunt down a custom PCM... I'm moving someplace far far closer to work so I won't have to worry so much about using expensive gas every day.