The control module of a car controls the engine's mixture of ingredients that are used to make energy, which are fuel, air and spark. It is able to do this by constantly monitoring the car's network of sensors to make sure that conditions are within the standard operating range.
When there is something wrong with a car, the control module first attempts to adjust the conditions. If it is unable to do so, the car won't start or run properly. In instances such as this, the control module stores a trouble code, and a mechanic can later diagnose the problem. The control module also activates the check engine light to let the driver know something is wrong.