A car's Check Engine light may come on when the oxygen sensor malfunctions, the gas cap fails, the ignition coil fails, spark plugs misfire, or the catalytic converter fails. Auto parts stores, repair shops and dealership service departments can check the car's computer codes to isolate the problem.
The oxygen sensor measures the amount of oxygen in the exhaust and tells the computer how much fuel to inject. A bad oxygen sensor sends inaccurate signals to the car's computer that hurts fuel efficiency. A loose, damaged or missing gas cap lets gas evaporate.
Ignoring the Check Engine light can lead to the failure of the catalytic converter, an expensive part that controls emissions. If the ignition coil goes bad, the car may not start. If the engine appears to misfire, one or more bad spark plugs can cause the Check Engine light to come on, meaning the engine is not operating efficiently.