Seems like every day someone brings a car into our shop that has a check engine light on (sometimes reads service engine soon or is a picture of an engine). 
What is the check engine light telling you? Primarily it tells you that there is a fault in the emission control system. The light is coming on so you know service may be necessary to protect the catalytic converter. Why do I say service may be necessary? Something as simple as a loose gas cap which causes a vapor leak in the fuel system may be the causing the light to come on. So the first thing to do if the check engine light comes on, and the car seems to be driving fine, is to make sure your gas cap is tight. If the light stays on it is best to bring the car to a repair shop to see what is causing the light to come on. Just ignoring the light may result in further, more expensive damage such as needing to replace the catalytic converter.
What are the most common repairs needed when the check engine light comes on? In our experience they are:
- Vapor leak in the fuel system.
- Failure of one or more oxygen sensors
- Vehicle needs a tune up (spark plugs, wires, etc).
- The mass air flow sensor needs cleaning or replacement.
If the check engine light comes on and blinks your car will probably be running very poorly. Immediately pull off to a safe spot and have the car towed to a repair shop as damage to the catalytic converter is likely to occur.
In Nevada, if you have a light duty vehicle that is 1996 or newer and the check engine light is on, it automatically FAILS THE STATE EMISSION TEST! If you ask for a smog check and the check engine light is on, all smog shops are required by law to do the smog test knowing that it will fail. If you tell the technician that your check engine light is on before asking for the smog test, then the shop can advise you how to proceed without wasting money on a failed smog test.
Some have tried to disconnect the battery to reset the check engine light before having it smogged. While this will turn off the light temporarily, a series of monitors need to be run before the vehicle can be smogged. In most cases the check engine light will come back on before the monitors run and the smog test can be performed.