Why does the engine fault light come on when the car is not ignited?
2 Answers
The engine fault light may come on when the car is not ignited due to the following reasons: 1. Faulty air flow sensor. If the air flow sensor or its circuit malfunctions, the ECU cannot receive the correct intake air signal, leading to improper fuel control and resulting in either too lean or too rich fuel mixture, causing the engine to malfunction. 2. Spark plug carbon buildup. Carbon buildup on spark plugs can cause poor engine performance, including difficulty starting, unstable idling, poor acceleration, backfiring during sudden acceleration, excessive exhaust emissions, and increased fuel consumption. 3. Engine knocking. When the engine inhales the air-fuel mixture, if the mixture ignites spontaneously before the compression stroke reaches the intended ignition point due to uncontrolled factors, the resulting shockwave opposes the piston movement, causing engine vibration, known as knocking. Symptoms include weak car performance, excessive engine noise, and, in severe cases, knocking sounds. 4. Damaged coolant temperature sensor. The coolant temperature sensor contains a thermistor with higher resistance at lower temperatures and lower resistance at higher temperatures. When the engine is cold, the sensor sends a signal to the ECU to enrich the air-fuel mixture for stable operation. If the sensor fails to send this signal, the mixture becomes too lean, causing abnormal engine operation. 5. Engine misfire. An obvious misfire leads to increased fuel consumption, black smoke emission, poor acceleration, and severe shaking.
Every time I turn the car key to the ON position without starting the engine and see the check engine light come on, it feels like a little surprise. Actually, that's the dashboard running a self-check—the light indicates the system is testing all functions, and it should turn off normally after ignition. If it stays lit, then there's a problem. Possible causes include the engine computer storing trouble codes (like a faulty oxygen sensor or clogged air filter) or an aging battery with unstable voltage triggering the warning. I often remind friends never to ignore it, as small issues can pile up and lead to expensive repairs. It's best to buy an OBD scanner to read trouble codes yourself—simple and cost-effective. Regular vehicle checks can prevent these headaches, ensuring safe and worry-free driving.
