Thread: P0420 Code keeps setting on 2001 GTP

Hi there! First post for me, so I hope to make it an interesting one!

My 2001 GTP has begun to set the P0420 Cat Efficiency code. Of course when I reset the PCM all the Readiness Monitors get reset too, which is a problem when I take it to testing for Emissions. The vehicle has 124K miles on it and very well maintained and in excellent running condition. It started setting this code about 2 months after I put a new Catco cat on with Bosch O2s. Initially I'm thought it could be either the cat or the O2s, so I replaced them both, the cat is now a Magnaflow and the O2s are GM. The rest of the exhaust is stock along with the engine except for a 180 thermostat & 3.4" pulley.

So, enough history, here is the data. According to my Snap On SOLUS, the code gets set after 100 miles or so of daily driving. It is set at idle, in drive, at 0MPH, minimal LT & ST trims, 180 ECT, 10 to 13inHG MAP, TPS @ 0% and calculated A/F at 13:1. What is wierd about the data is the MAP value and A/F ratio. Normal running when I check it, the MAP is 18inHG or higher with a 14.7:1 A/F. I can't figure out why it would only set this code after driving a while, several ignition cycles later, and how it correlates to the low MAP and rich mixture values. I'm hoping with all the smart people on here that someone might have some ideas. Anything is worth looking into.

Thanks a lot for reading this and helping out a fellow GTPr

Thank you for your input. The PCM senses the catalytic being there since I can watch the cross counts and input voltage from the rear O2. It is a very specific condition as to when the code is set, that being at idle, in drive, 0% TPS, etc. I have been running the minimal vehicle mods for 120K miles, no issues until now. It does have an ASCD ram air hood with the SLP ram air cold induction kit. Normal improvement stuff like MSD 8.5mm wires, Gatorback belts etc. Either the PCM input has gone south or the PCM wants to see a specific mixture signal during this time. Watching an O2 simulator versus the real O2, I can't see the difference. This one is more complicated than most.

Thanks for the thoughts. I'll try some more poking around.