How to reset check engine light

Don’t reset your check engine light

You likely found this page trying to find out how to reset your check engine light. I’ll tell you, right at the top of the article, but please read on to find out why you shouldn’t.

To turn your check engine light off:

  • Disconnect your 12V battery for 15 seconds. This will turn the check engine light off and clear the computer’s memory, or
  • Buy a scanner from Amazon, EBay, or an auto parts store and use it to clear the trouble codes, or
  • Find the fuse that supplies backup/memory power to the ECM and pull it for 15 seconds.

Why you shouldn’t just turn off your check engine light

If there’s a problem, the light will just come back

When you reset your computer’s memory, the warning light will turn off. However, if there’s a problem, the light will come back on as soon as the computer runs the test that caused the light in the first place. It might happen right after you turn the car on, which might make it seem like you can’t clear the check engine light. Or it might take a couple of days to come back.

Clearing the check engine light can cause your car to fail smog

If you’re trying to pass a smog test, clearing the check engine light isn’t a fix. Yes, a car will automatically fail a smog check if the check engine light is on. However, clearing codes won’t make it pass. To pass smog, the “monitors” must be complete. Monitors are the intermittent tests the computer runs. Some tests are run while the car is driving on the freeway. Some are run when the car is sitting parked. Some require the car sit while idling. And some require all of the above. If you clear the check engine light and go for a smog check, the car will fail.

“So what” you might think, “I’ll clear the check engine light and then drive around so the monitors will run, then I’ll go for a smog test.” Well, if nothing is wrong with the car, this will work. However, if the monitor runs and passes, the check engine light will turn off by itself. No reset is required. Most of the time, there’s a problem when the check engine light is on.

When you clear codes, you also clear diagnostic data

When the check engine light comes on, a trouble code is stored and freeze frame data is stored. A trouble code lets us know which computer test failed. The freeze frame data shows us the data the computer saw at the time of the fault. Resetting the check engine light erases all of this data. Without this data diagnosis is more difficult.

When the check engine light comes back, new data will be recorded, but you never know if it will be as good as the original data. Think of it like taking a pictures of a group of friends. You take a bunch because some will be good and some won’t. Freeze frame data is often the same. You don’t know which snapshot will be best, so you look at all of them before deleting.

Here are some other related articles:

Should you stop driving if the check engine light comes on?

How to get a catalyst monitor to run