Though it's not always the case with the service engine light, most often the source of the light is the crankshaft sensor, which is a three-wired sensor shaped like a cylinder facing the engine's firewall, located just below the exhaust manifold. First check the wires and make sure they're all connected to their respective leads. If the sensor has been in place for upwards of 50,000 miles, it's probably time to replace it. The sensor is held in place by 10mm bolts. Unscrew them and fit a replacement sensor into place.
These lights mean that the car's automatic braking system and traction control aren't working. Often with the Pontiac Grand Prix, this will happen even though these systems are working just fine. Another symptom is that the power steering will stiffen, making turning very difficult. Most often this is a result of an ABS speed sensor located in the wheel hubs being broken or so covered with dust and debris that it can't take an accurate reading from the tires as they rotate.
Put the car on jacks and remove the wheels. Examine the wiring leading from the sensors, and clean the lenses on the sensors with a non-oil-based solution (Windex works fine). If the lenses are clean, it's likely the sensors themselves will have to be replaced. Once replaced, the car's computer error codes have to be reset. Unhook the battery for about an hour to force the computer to reboot when you next start it. That should take care of the problem.
The Pontiac Grand Prix often has problems with its ignition module, which is a very complex system that only a trained mechanic can look at, though there are a few things you can check to make sure are working properly first. Unhook the battery to safely remove the fuel injection fuse from the fuse box. Unscrew each spark plug in turn from its port in the engine, and place them against a grounded metal surface. Have someone help crank the engine. Each plug should generate a blue spark; replace any that don't. Beyond this, the only thing that can be done is checking the ignition coils. Disconnect one and clip an Ohm meter to it. There should be a steady, low level of resistance. If not, the coils need to be replaced as well. If this doesn't help, the car is best taken to a mechanic.
