filled up gas tank and p0420 code came on
P0420 code (Catalytic Converter) can be generated by any number of sensors upstream of the converter. More than likely there is a problem with the evap recovery system or the valve that's supposed to prevent over-filling the tank. When this happens, excess gas fumes are drawn into the engine (as it should) causing a rich condition, which triggers the Cat sensor to throw the P0420 code. Get the tank filler pipe and evap system checked out. There is probably one or more other codes being thrown, check the entire system.
You may just have a loose gas cap. When I worked in parts, I had that code and complaint all the time and 95% of the time it was cause of a loose gas cap.
Looking for a Used Grand Prix in your area?
CarGurus has 4,288 nationwide Grand Prix listings starting at $1,000.
Search Pontiac Grand Prix Questions
Pontiac Grand Prix Experts