1. One Sensor has a Loose Electrical Connection or is Damaged:
Believe it or not, the most common reason the CEL will light-up is due to a poor electrical connection from the ECU to the sensor. When the electrical harness is loose, frayed, damaged, or exposed, it can send a signal to the ECU and mimic a mechanical problem. It's also common for sensors to wear out over an extended period of time. This tends to happen quicker with fuel and exhaust system sensors, which are exposed to hot gases and carbon deposits.
2. Damaged O2 sensor:
The emissions system on your vehicle is highly complex. It starts with a series of sensors that monitor the fuel/air ratio and the amount of carbon deposits coming from your exhaust. Since these sensors are exposed to hot exhaust and can be corroded with carbon deposits from fuel and exhaust vapor, they can become damaged rather easily. When the oxygen (O2) sensor fails, it can cause the engine to misfire, lead to poor acceleration, bad fuel economy, and more. An O2 sensor failure will typically cause a vehicle to fail an emissions test in most US states.
3. Catalytic Converter is Clogged:
While the O2 sensors monitor exhaust gases, the catalytic converter is designed to filter exhaust before heading out the tailpipe. Like any other filter, the catalytic converter can wear out or become clogged with too much debris to be effective. When this happens, it will create an error code and trigger the CEL. A clogged catalytic converter can cause poor fuel mileage, engine misfiring, bad acceleration and even lead to internal engine damage if not replaced.
4. Loose or Damaged Fuel Cap:
Today's modern cars, trucks and SUV's are very carefully monitored for fuel economy and emissions. If the gas cap is either left off, damaged, or does not maintain a solid seal, it will create less pressure inside the fuel cell. This tells the sensor there is a problem, creates the error code, and - you guessed it - illuminates the check engine light. When the fuel cap is loose or damaged, it can cause acceleration problems and/or poor fuel economy.
5. Mass Air Flow Sensor is Damaged:
You're probably seeing a pattern here. In reality, most check engine lights are triggered due to fuel or emissions system issues. The mass air flow sensor or MAF is responsible for carefully mixing the right ratio of air to fuel, maintaining a clean and efficient burn inside the combustion chamber of your engine. If the MAF is damaged, the MAF sensor will send a signal to the ECU to let you know this needs to be replaced or cleaned. A faulty mass air flow sensor can prevent your engine from starting. If your car will crank over but not light-up, this is probably the source.