"check engine" light

[tmleadr03]

On the newer ones you need to turn the lights on and try to start it with the battery disconnected or wait a really long time to fully drain the system. Otherwise it'll hold the information. Also car may run rough for a minute or gas mileage drop for a short time when it restarts after this as the computer needs to relearn what setting it had before.

OBD scanners are so cheap now that it's almost pointless to not have one though, and then you can reset it yourself w/o all the hassle.

I have had cars come in with codes, disconnected the battery started taking stuff apart, waited on parts and days later got the vehicle back together and the codes were still in the system.

If you turn off your PC computer do you lose all the information stored on it?

[BigNick73]

I have had cars come in with codes, disconnected the battery started taking stuff apart, waited on parts and days later got the vehicle back together and the codes were still in the system.

If you turn off your PC computer do you lose all the information stored on it?

Well I'm not a professional mechanic, but I've got a 07 jeep and 08 Subaru. On either one if I d/c the battery, wait 10 minutes and reconnect the CEL will be there when I restarted, but hit the lights or ignition and really drain it the CEL would clear. Now if it was a legit problem it'd pop back up, maybe after a few seconds, maybe after driving for a day. The POS jeep is bad about having random issues from elevation changes, like vail pass. I've reset it on the side of the road a few times using that method since sometimes it'll actually effect how it's running, but d/c the battery, turn the key with the lights on, reconnect batt. and it'll go back to normal, and CEL won't come back.

Also had to use that method for clearing codes on a 97 f150 I had years ago (first OBDII vehicle I ever owned). Had a vacuum leak that was giving me a lean code and wouldn't clear on its own even after it was repaired, seemed easier than driving back to napa to use the scan tool.

The way I understand it the control module has a capacitor in it that supplies enough power to hold the information, and it can hold out for a very very long time, that's why you turn on the ignition or something else to drain it. Kind of the same way your BIOS settings on your PC stay even with the power off until that little button battery on the motherboard goes dead and then you're redoing all the settings every time you turn it back on.

I could be way off on all of this but it's how I've always understood that stuff to work on the newer cars. The really new stuff might have some sort of solid state memory that doesn't need power.