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
