First post ............

Car is Pontiac Grand Prix 2006 (base model).

So got codes:
P0442
P0455
P0137
P2A01

It started with an old gas cap with a worn down seal, thought that replacing it with an aftermarket one would fix it but not the case. The first two codes have to do with the evap system and gas cap (hopefully just the gas cap).

The later two codes are the rear O2 censor. Tomorrow I'm going to get a new one and put it in.

I'm trying to fix up the car to pass a safety inspection so I figure it shouldn't have a Check Engine Light just in case they'll look at it.

Anyone have similar issues with their Grand Prix or dealt with this before?

I can't spell ..... should say sensor not censor .............

for the first two, i would think your vent valve is stuck open, you can take it off and test it. its sort of by the fuel filler on the inside of the fender. you can take the filter off and blow through it, if you apply battery voltage one way you will be able to blow through and the other it should seal. they usually hang up visually if the solenoid is still functioning you can squirt some wd40 in it and actuate it a few times and put it back in.

the last two could be circuit issues with the sensor you have access to drawings for the rear o2? you should check for voltages at the plug and the wiring to the pcm, if those check out it may be the sensor