Hello friends!
My 2005 Pontiac Grand Prix GP gave me a PO420 about a year and a half ago. I replaced both the O2 sensors and the catalytic converter (although the more time goes on, I think I just had a bad 02 sensor...keep reading and you'll see why). The check engine light went away for maybe a month and then it came back throwing the same p0420. Every now and again it would go off and then come back on, throwing the same p0420. It was off almost all the time this summer and then once the weather got cooler, it came back on. In addition to the p0420, I now have a p0054.
Do you think I got a faulty downstream O2 sensor? Is my wiring bad going to the 02 sensor? Any ideas on how to diagnose for sure what is wrong? My gas mileage has gone to crap all of the sudden and I think it coincides with the p0054.
