Q: Fixed car after Service Engine Soon light came on. It's still not working right

I can understand how frustrating it can be to replace parts on your car only to have it continue to experience problems. There can be several reasons why your Pontiac has its’ Service Engine Soon Light illuminated. If there is a misfire present after performing a basic tune-up, which includes replacing the spark plugs and ignition wires, then next we need to determine what else could be causing your Pontiac’s problems. I would first start with reading the computer on the vehicle. This is best step towards verifying which codes may be causing the SES light come on. From my experience, it is also possible that your Grand Prix may be having fuel issues, perhaps due to a lack of fuel pressure or a problem with one or more of the fuel injectors. Another common possibility is that the misfire is being caused by internal engine damage, such as a bent valve. With a sensitive issue like this, I find it best to enlist the assistance of a certified mechanic to come out and determine the fixes for your car’s Service Engine Soon Light.