If you have had your car or SUV long enough, that annoying "check engine" light has probably made an appearance on your dash board. The first time I saw it, I popped the hood and "checked" the engine. It seemed ok to me. My car was running fine. So like most experienced car drivers, I ignored it or I tried too. The really insidious thing about that little innocent light is that it really gets to you after a while. After a long drive at night with that light shining at me like a laser beam, I considered putting it out its misery with my tire iron. Which brings up the question, what is it there for. Well here is a basic answer. The "check engine" light is connected to the car's engine fuel management computer. This is the computer that runs the fuel injectors, reads the engine sensors, etc. The oxygen sensors are used by the ECM to determine how accurately it's injecting the fuel, and adjusting the way the fuel is injected to compensate for changing conditions. When the computer inside the ECM detects that there is something wrong, the ECM turns on the "check engine" light. For example, a bad sensor or engine operating parameter that exceeds a preprogrammed set point then the ECM turns on the light to let you, the driver, know that something is wrong and needs to be corrected. It doesn't mean something cataclysmic is about to happen, just that the computer that runs a lot of the functions in your engine is getting some bad information. Most of the time this means an oxygen sensor has failed and needs to be replaced. In a lot of states, your car will not pass the inspection when the light is on. So you have to fix it before you can get a new inspection sticker.