Code 0420

2002 GPGT 3.8 vin K. I got a 420 code that won't go away. I've replaced both O2 sensors(generics from RockAuto and then with Bosch) and also the convertor, upper and lower intake gaskets with a new plenum and all the related gaskets. plugs/wires are recent. no drivability issues. I clear the code...It comes back in 10-14 days. A buddy of mine said he had the same issue with a Toyota at his garage. It took original dealer O2 sensors to correct.

Thoughts and Suggestions?

I would check to make sure the wiring going to the o2 sensors are in good shape and not burnt or broke at the connectors. Try to check as much as you can. I really doubt the sensors are bad though.

If you could find somebody with a tuner, you could get them to turn the rear o2 sensor code off and see if that fixes your problem.

There used to be a resistor mod that you could do for the rear o2 sensor but I never could get it to work. I also think there are o2 simulators(might be called something else) that fake the computer also.

Sometimes, you can replace the cat with a high flow and they will also cause you to throw a code. I could never get our code to quit from coming on and I bought a dhp 1.0 pcm that had the o2 code turned off and fixed our issue.

Good luck with your issue though.

get a 90 degree oxygen sensor extender off ebay for $15. Worked on both my cars that got P0420 codes after adding high flow cats.

crying shame this post is already a year old, man.