Modern cars are run by efficient computerized systems that constantly monitor the vehicle's performance. When a malfunction is detected, the check engine light (CEL) will come on in the dashboard to let the driver know a problem has been detected. Unfortunately, the light itself doesn't tell the driver what the problem is — it could be anything from a loose cap on the gas tank to a problem with the catalytic converter. Fortunately, when the check engine light is triggered, the engine control unit (ECU) will log one or more codes that can help point mechanics toward the source of the problem.
A check engine light doesn't always mean a big problem or an expensive repair is needed. However, driving a car with the light on could mean that you are doing further damage to the engine, depending on the problem. It's always best to get the vehicle checked at your earliest convenience.
Loose or Missing Gas Cap
If the vehicle's gas cap is loose or missing, the car's computer can register this as a problem with the emissions system. Gasoline from the tank may evaporate out, lowering the mileage on the car. This problem is usually very easy to fix. If the cap is loose, the check engine light should go off a day or two after it's tightened. Replacing a cap is also very inexpensive; some auto repair shops may even supply a new one for free.
Electronic Fuel Injection System Problems
Engines encompass a number of systems that work together to provide excellent performance under normal circumstances. The vast majority of modern cars are powered by an electronic fuel injection (EFI) system, which uses a number of valves and sensors to ensure the correct amount of air and fuel are fed to the piston chambers to create the right combustion mixture. The mass flow sensor and oxygen (O2) sensors are key parts of this system, and a malfunction in either can cause the light to come on. Problems in the EFI can affect other systems, including the catalytic converter, so it's important to fix them right away.
Worn Out Spark Plugs
Timing of the spark is also controlled electronically. If the spark plugs are worn out or misfiring, they can cause the car to jolt during acceleration. Bad or broken wires can also cause misfires. Both the spark plugs and the wires are relatively easy to replace, and can improve the car's performance.
Vacuum Leak
Another event that is often responsible for tripping the check engine light is a vacuum leak. A hose or manifold might be loose or cracked, which will disrupt the fuel/air ratio in the engine. A visual inspection can help, but in some cases a vacuum leak can be very difficult to locate. Some repair shops offer a smoke test, in which the air intake system and hoses are filled with smoke, and then closely monitored to see where it escapes. Vacuum leaks that can trip the light include a sticky exhaust gas recirculation (EGR) valve, or more seriously, a blown head gasket.
Catalytic Converter Problems
The catalytic converter is part of the emissions system, where it converts the gasses created by combustion in the engine to less harmful exhaust. In cars that are maintained regularly, this part rarely fails. If other parts, like the O2 sensors or spark plugs, malfunction and are not replaced, they can damage the catalytic converter, however. This is a very expensive repair, but the car cannot run without it.
Diagnosing the Problem
There are so many possible reasons for the check engine light to come on, so using a reader to retrieve the on-board diagnostic (OBD) codes can be helpful when the problem is not immediately evident. Even if the light is only on a short time, a code will be stored in the ECU for later retrieval. In most cars, it is easy for a mechanic or even the car's owner to attach the code reader to the appropriate port and get this information. The code can then be used to determine the problem.