A quick guide:
The "Check Engine" or ("MIL") light indicates that your engine computer has detected a fault in one of the engine controls or sensors, especially one of the controls related to emissions control. Examples include intake air temperature sensors, knock sensors, and oxygen sensors.
- If your MIL is flashing, STOP DRIVING IMMEDIATELY (IF IT IS SAFE TO DO SO). Your engine is misfiring at a high rate. You can damage your catalyst and even your engine if you drive in this condition for more than a short time. Pull over and have the car towed to the nearest mechanic or to your dealer.
- If your MIL is lit, but not flashing, drive normally. You won't harm your vehicle if it is running smoothly. When you have time, pull your battery cable, or the fuse which feeds your engine computer. Wait 30s, and then re-connect it. This will reset your computer. If you have a real problem, the MIL will come back, usually in several days.
- Most problems that cause the MIL to light are not dangerous to drive with, however, you may be polluting the air a little more than the EPA or CARB would like. You also may find you have reduced fuel efficiency, rough idling, or reduced power.
- Probably the most common cause for a MIL is a loose gas cap, which triggers an evaporative emissions system code. If you get a MIL, try tightening down the gas cap, and disconnect the battery as mentioned above.
If you are so inclined, you can get an OBD code scanner for less than $100, and read your diagnostic codes yourself. Then, you may be able ot pinpoint and repair your problem yourself. There is a list of OBD-II codes here.