After re-tightening the gas cap:
* Drive the vehicle for a day or two. The light might turn off automatically after the system detects no further leaks. Many OBD-II systems will clear a gas cap-related code after a certain number of successful drive cycles.
* If the light *remains* on: The problem is likely something more serious than just the gas cap. You'll need to have the code read by an OBD-II scanner (many auto parts stores offer this service for free). The code will tell you exactly what the car's computer detected. This is crucial before you start throwing parts at the problem. Guessing can be expensive.
In short, while a tight gas cap *might* solve the problem, it's important to get the code read to determine the real cause of the check engine light. Ignoring it could lead to bigger problems down the road.
