Recent thread: http://boards.straightdope.com/sdmb/showthread.php?p=11712959#post11712959
Is this a mandatory feature in the US only? Because leaving the cap open in a Euro-spec car won’t turn on the “check engine” light.
It’s not mandatory, but it’s a side effect of the other emissions controls that are mandatory. It’s basically fooling the emissions controls into thinking there’s a leak in the system somewhere. The emissions system doesn’t realize that it’s just the gas cap off that is causing the leak.
Stupid emmisions system! 
The one and only redeeming feature of my Ford Windstar was that it had a light on the dashboard that said “Check Gas Cap” which came on if the cap was loose. I wish other manufacturers would offer this, but then the dealers wouldn’t get paid $100 to tighten the gas cap and reset the computer.
I believe that a lot of the auto supply chains (Pep Boys, O’Reilly’s, AutoZone, etc.) will check the error code for free.
IIRC, they are not supposed to reset the error code for you. But some of them will, or will tell you how to do it.
Yes, I had to pay the dealer for this once. It was only $35 back then (one hour of service). It wasn’t me, it was my SO. But I didn’t remember that I’d been warned about this problem, so when the light came on, I went to the dealer. Live and learn.
Man, that “check gascap” light would be bitchin!
Doesn’t the check engine light go away if you fix the gas cap. I am pretty sure I read that in the manuals for my current car and the one before it. There is no need to get the light reset if it was that loose gas cap that caused it.
If you do some googling, you might be able to find instructions on how to read the code and reset the computer yourself. In my car it involved some sequence of stomping on the gas pedal a certain number of times within a certain interval, then waiting a specific number of seconds, then holding down the gas pedal for a certain length of time, etc. The code was then displayed by blinking the odometer in a certain way. It told me my gas cap was loose :-/
Don’t the caps all rachet nowadays? Even my previous truck, a '97, had a ratcheting cap.
They do, but it’s still possible to get it threaded wrong so that it doesn’t tighten up properly.
Sure, but even then they may not work as intended. Has that happen and Autozone hooked up to the computer and got a meaningless error code back. Bought a $10 gas cap, put it on, and within a mile or two the check engine light went away.
On a similar note, my Saturn has a change oil light. It is reset by a code of gas pedal stomps. Which I have to look up every time.