Check Engine Light: A basic question
 

Shouldn't a check engine light stay illuminated until the offending code is cleared? What would cause the light to come on, then abruptly go out just a couple of miles down the road?

Okay, so that's actually two questions...

dcb

Re: Check Engine Light: A basic question
 

Yes, if you have a foult.

A guess: the lambda.

http://www.subaru-svx.net/forum/showthread.php?t=54033

Re: Check Engine Light: A basic question
 

I have a CEL that comes on sometimes when I start up the car then goes off a little ways downt he road. When I pulled the code it was EGR issues.

Re: Check Engine Light: A basic question
 

Here's the simplified version:

Each sensor is tested several times a second.

One bad result is usually ignored, but if you get several bad results in succession the ECU will decide there is a problem and trigger the light.

If you then get a whole load of good results without any bad ones then the ECU will decide the problem has gone away and turn the light off again.

Re: Check Engine Light: A basic question
 

Thank you. I figured maybe it was the budget, no-name, 87 octane, petro-byproduct-laced "gasoline" with which I topped the tank before commencing a 2-hour highway run Sunday night that had my engine scratching its head. Haven't seen the CEL again.

dcb

Re: Check Engine Light: A basic question
 

I also have had this cel situation, I believe it caused by old hard vacuum lines, always goes off though..........G.