Why does the check engine light come on and how do you reset it?

The "check engine light" is a warning light that is illuminated when there is a problem with the EMISSION SYSTEM only. Emission system being the pollution control system. It is a good thing once you understand it.


The " check engine light" is also known as a malfunction indicator light (MIL). You can disconnect the battery red lead for about 5 min and it will reset. However, if there is a hard fault, it will come back on.


Disconnecting the battery won't do the trick on OBD 2 vehicles.


Without a scan tool designed to reset the electronic control module (ecm) a do-it-your-selfer can't reset the check engine light.

Pull the codes from the computer using an OBD (II) code reader. Match the code to the troubleshooting procedure, follow the procedure to find the source of the fault. After you have repaired the fault, the light will go out if that was the only problem. There are "monitors" or self tests the computer runs the car through a drive cycle, if a problem occurs, it may not run all of the self tests until that problem is taken care. Therefore, another problem may exist.


One point that was brought up a a recent meeting of technicians was that the amount of hydrocarbons is greater when the gas cap is left off than when the engine is running. Hydrocarbons are part of pollution emitted as gasoline evaporates. Going a step farther, one facet of the emission system is the "Evaporative" portion. This is when the fumes from the gasoline are leaking from the system into the outside air. This is one part of the emission system that can trigger a check engine light. I would say that about 7% of the vehicles that have a check engine light are the result of a loose or inadequate gas cap.

But understand that many scenarios are possible with the "check engine light" The vehicle's powertrain computer (note that some vehicles have 17 different computers) will run a series of self-tests. They will only run under certain criteria and they can be vastly different from manufacturer to manufacturer. Some self-tests are not run until preceding ones have run successfully. So if there is a problem in one particular area that is preventing another self test from running, you can have a situation where one problem is fixed, but another still exists. If you fix a problem and drive the car through a drive cycle that sets the monitor (or self test) the light will go off as it passes that criteria that triggered it in the first place.

After 1996, the auto industry went to a idea called OBD II (On Board Diagnostics). This was to get all the manufacturers onto a similar plane for troubleshooting and powertrain control. While they still differ vastly, many corrections and adaptations were made for technicians to better fix the check engine light problems.

Prior to this there were so many different and poor troubleshooting data from a check engine light problem that resolving the problem was much more difficult.

Many early warning lights of this nature were set to illuminate based on mileage. An oxygen sensor was one of the things that was meant to be replaced when that mileage was hit. This is much like many current "Change oil lights" which are set up to come on after a pre-set mileage.


Great question and good answer above. There is one thing to add however. On my 2004 Buick for example I am required to manually reset the "check oil" light myself every time I get an oil change. This ensures that the system understands the condition the oil is in. If/when I do not reset it, not long after my expected date the light goes on by default as a reminder.

Your instruction manual would outline this. Mine for example is simply...

1. Insert key

2. Turn key to ON, without engaging engine (just before the engine cranks, and leave key there)

3. When placed to ON, press and depress the brake slowly 5 times in 5 seconds.

4. Turn key to off.

5. Turn key and start engine.

The check engine light is now reset, if it returns shortly it is an actual issue, rather then a reminder.


Some Ford Mondeos have a 'Service interval expired' warning light. It is pretty much the same as the "check oil" light mentioned in the answer above.

On that type of Ford Mondeo there is a small hole in the display screen in front of the 'Service interval expired' warning light. To reset it, just turn the ignition key until the all warning lights come on but do not start the engine. Then press a tiny button in the warning light display panel by poking a straightened metal paper clip through the hole. If you hold it steady for 10 seconds whilst keeping the ignition on, the warning light goes out. It will come on again when the service time period again expires.