Why does the fault light come on after the car is powered off?
3 Answers
Most fault lights will disappear after startup, because when the car is powered off, the corresponding computer board of the car will record this situation as a fault code and store it. Therefore, after power is restored, the corresponding fault light will illuminate. You can use a car diagnostic tool to read the fault code and clear it. If it cannot be cleared, matching is required. If it still cannot function normally after matching, there may be an existing fault that requires troubleshooting. The general reasons for the car fault light coming on are as follows: 1. Poor fuel quality: If the engine fault light on the car dashboard comes on shortly after refueling, it is usually due to incorrect or poor-quality fuel that does not meet standards, resulting in insufficient combustion of the air-fuel mixture during engine operation. 2. Oxygen sensor failure: Currently, cars are generally equipped with two oxygen sensors, one before and one after the three-way catalytic converter. Their function is to detect the air-fuel ratio under different engine operating conditions. The front one provides signals to the ECU to adjust the fuel injection amount and calculate the ignition timing, while the rear one is used to detect the performance of the three-way catalytic converter. 3. Air flow sensor failure: If the air flow sensor is not working properly, the ECU will not receive the correct intake air volume signal, leading to problems similar to those mentioned above, and the engine will not operate normally.
When the car lost power, the warning lights came on. I've experienced this myself. It might be that the car's computer is too sensitive after rebooting and temporarily reports an error code; or the battery connection is loose, causing unstable voltage and sensor misjudgment. For example, last time I replaced the battery, all the warning lights flashed, but they turned off automatically after driving for a few minutes. I suggest you first check if the battery terminals and wires are corroded or loose; then try starting the engine and driving a short distance. If the lights don't go off, there might be a real issue, like insufficient power generation from the alternator or a short circuit in the wiring, and you should go to a 4S shop immediately for professional diagnosis. Don't underestimate these warning lights, especially after a power loss, as they indicate potential problems that could cause more serious damage if ignored. Regular battery maintenance can prevent similar issues, such as checking the voltage periodically.
I often repair cars and know the reason: after a power outage, the fault light usually comes on due to voltage fluctuations during system reset, causing the ECU (Engine Control Unit) to misinterpret fault signals. Common triggers include corroded battery terminals or low voltage triggering protection. The steps to handle it are simple: check if the battery terminals are tight; use a multimeter to measure the voltage—if it's below 12.6V, charge it. Start the engine and drive around for a while, waiting for the light to turn off by itself. If it remains on, don’t tamper with it—connect an OBD scanner to read the trouble codes and identify the root cause, which could be an alternator or sensor issue. Delaying repairs can worsen the damage, so act quickly to ensure safety. Developing a habit of performing a thorough inspection after every power outage can save you a lot of money.
