My 2007 Grand Prix has a recurring P420 engine light code, meaning a bad catalytic converter (or bad O2 sensor?). In CA, cats are replaced by dealers only for $1400. I cannot buy a new one for less than $1100 unless I travel out of state and bring one back. I have tried lacquer thinner in the gas, and fuel system additives, but the light keeps coming back on. Should I try a new O2 sensor? Defouler? Any other suggestions? The engine runs great. I replaced the spark plugs and wires, and have a new radiator with clean coolant.