Maybe you’re driving, and the check engine light comes on. The best thing to do is pull over and have a code reader relay the trouble code to you. This way, you’ll be able to determine if the issue is something you can fix yourself (a loose gas cap). If you can fix it by yourself, you can do so. You're off the hook if the light doesn’t come on again! There are situations where you do not have a code reader. Maybe the trouble code given to you was something you could not fix by yourself. It will be best to pull over. Stop the engine, and call the nearest repair shop. Stay calm. Do not drive while the light is still on.