Check Engine Light is On – Possible Reasons

Loose or Faulty Gas Cap is the most common reason. A loose or damaged gas cap can trigger the check engine light by allowing fuel vapors to escape. Also check for the smell of gasoline vapors around the gas cap.

Oxygen Sensor (O2 Sensor) monitors the level of oxygen in the exhaust gases. A malfunctioning sensor can lead to reduced fuel efficiency, increased exhaust emissions, rough or uneven idling, and sluggish acceleration.

Faulty Mass Airflow Sensor (MAF) measures the amount of air entering the engine, helping to adjust the fuel-air mixture. A faulty MAF sensor can affect engine performance and cause hesitation during acceleration, stalling or rough idling.

Catalytic Converter problems can trigger the check engine light. This is a critical component for reducing emissions, and a malfunction can result in reduced fuel efficiency, reduced engine power, rattling noise from the catalytic converter, or a rotten egg smell from the exhaust.

Ignition Coil and Spark Plug issues can cause engine misfires, poor acceleration / power, rough idling, and increased fuel consumption – leading to the check engine light coming on.

Engine Vacuum Leak in the intake system can disrupt the air-fuel mixture and cause the light to illuminate. Symptoms would include: a hissing or whistling noise from the engine, rough idling, stalling when idling, and/or poor acceleration.

Exhaust Gas Recirculation (EGR) Valve can affect engine performance, exhaust emissions, rough idling, or even engine knocking or pinging – leading to a check engine light.

Faulty Thermostat can lead to engine overheating or running too cold, poor cabin heating or cooling, and poor fuel efficiency – any of which triggering the light to come on.

Evaporative Emission Control System (EVAP) controls the emission of fuel vapors. A loose gas cap is one issue, but there are other components like valves and hoses that can malfunction. This is cause the check engine light to come on. Signs you have an EVAP problem include: gasoline odor around the vehicle, problems filling the gas tank (fuel nozzle constantly clicks off), lower fuel efficiency, or increased emissions.

Transmission issues can trigger the check engine light, especially if the car’s onboard computer detects abnormal transmission operation. If you notice the transmission slipping or hesitating, harsh or delayed shifting, or transmission warning lights in addition to the check engine light, then the transmission could be the culprit.

Knock Sensor detects engine knocking or pinging. A malfunctioning knock sensor can affect engine performance and trigger the light. If you engine is knocking or pinging, then the knock sensor itself is probably working and you should examine the other possible causes for the check engine light to light up.

Fuel Injector problems can lead to poor engine performance, such as rough idling or engine misfires, which will cause the check engine light to come on.

Camshaft or Crankshaft Position Sensors help the engine control module (ECM) manage ignition timing and fuel injection. If these sensors malfunction, then the engine may not start at all or it might stall frequently.

Issues with the Exhaust Gas Sensor (EGS) monitors exhaust gas temperature and can lead to emissions-related problems if it fails. 

Engine Control Module (ECM) or Computer can have issues that cause the check engine light to come on. Look for erratic engine behaviors, other warning lights, and drivability issues.

Various other sensors in the engine or emissions system can fail and trigger the light, including the throttle position sensor, coolant temperature sensor, and more.

When your check engine light comes on, have a diagnostic scan performed to retrieve the specific error codes. These codes help pinpoint the exact issue and help a mechanic or technician diagnose and resolve the problem. Ignoring the check engine light could lead to more significant and costly issues down the road.

This table presents the common signs or symptoms that may accompany each reason for the check engine light (CEL) to come on. Please note that while these signs are often associated with the respective reasons, the absence of an “X” in a column does not necessarily mean the symptom won’t occur; it simply means that the symptom is less commonly associated with that particular issue. Symptoms can vary depending on the severity and nature of the problem.