Check engine lights go on when there is a problem in the engine or the emissions control system. You should always have the codes generated by your car's computer scanned and read so you can determine the cause. You can reset the light once you have fixed the problem.
Check the Error Codes
When the check engine light goes on, an error code is saved in the car's computer. With the right tool, called an automotive diagnostic scanner or a code reader, you (or a mechanic) can read this code and find out what's wrong with the vehicle. Relatively inexpensive code readers are available, some for less than $100 US Dollars (USD), but an automotive parts store or repair shop may be willing to check the code for free. These readers may also be able to clear the code from the computer, in some cases, turning the light off quickly and easily.
Disconnect the battery
In some cars, especially those made before the mid-1990s, you can reset the check engine light by disconnecting the battery. Newer vehicles have a feature that will simply turn the light back on after the battery is reconnected if the car is not drained of electricity properly, however. You can do this by constantly blowing the car's horn until it dies or turning on the headlights, then waiting several minutes. This will ensure all electrical current has been drained from the car's capacitor. At this point, it should be safe to reconnect the battery and the check engine light should not longer be lit.
Disconnecting the battery will erase all the data from the vehicle's computer, which could make any problems with the car more difficult to diagnose in the future. It will also reset the electronic systems inside the car; if the car has an anti-theft radio system, for example, it will likely go into safe mode and will need a code to unlock it. All radio station presets will also need to be reprogrammed.
If you're trying to turn the light off in hopes of passing an emissions test, this method rarely works. Even if all emissions data is cleared from the computer when the battery is disconnected, the car's systems need to complete a series of tests called a "drive cycle" before it can pass testing; sometimes more than one drive cycle is needed before the car is ready. The chance of the car completing its internal tests and the light not coming back on before the emissions test on is very small.
Fix the car
The best way to turn off the check engine light is to fix whatever problem has caused it to come on in the first place. Even if you were able to turn it off by clearing the codes or disconnecting the battery, if what caused the light to come on hasn't been corrected, it probably won't stay off for very long. Postponing a repair by ignoring the car's warning is usually a costly mistake; a problem that could have been fixed easily early on could grow into a major repair if it's not addressed in time. Unless you're comfortable checking the error codes and doing repairs yourself, it's best to take the car to a mechanic and get it fixed.
Service required light
Sometimes, drivers confuse the "Service Required" warning with the check engine light. In most vehicles, these two warning lights mean different things. The service engine indicator means that the car is due for routine maintenance or an oil change. Its intervals are typically set by mileage, not because the car's computer detects anything wrong.
Erasing codes by disconnecting your vehicle's battery can also blow out the memory for your radio and other on-board components. This method is recommended as a last resort.
The emissions monitors in your vehicle will register being reset, so you will not pass emissions if you bring in a car with the codes being reset. Drive your car for at least 200 miles (320 km) before bringing it in for an emissions test.
Consult a mechanic or repair shop if the code keeps recurring or if you cannot find information on your code or how to fix it. A qualified mechanic might need to look at your car and reset the codes.
Reputable auto shops will not reset a check engine light if the problem is not fixed. To do so would be illegal.
Check engine light being on is a sign that there is a problem with the car. Resetting it without ensuring there is a problem can be dangerous.
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