For a number of months the "Check Gas Cap" warning message appears randomly followed shortly by the "Service Engine" light. It does not appear to be related to gas fill-uos, in fact occurring days and hundreds of miles after a fill-up. My mechanic scanned for error codes and came up only with a generic evaporative system code. Investigation of the system did not find any leaks or other obvious problems. I can clear the Gas Cap warning by disconnecting the battery for a few minutes but the service engine light stays on and finally goes out many days later. What are the chances it really is the gas cap? It is always tightened with many clicks at fill up and is never loose when the warning appears. If not the cap what else should we be looking for? The car is a 2002 Seville SLS with 85000 miles on the odometer.

Thanks!

Hey Nels,

My wife's 2004 Grand Prix was doing the same thing. Her car on had 38000 miles on it. I bought a new cap from the Pontiac dealer and the messages went away after 2 days of driving. Apparently the seal goes bad on the caps. It may be clicking when you are tightening it but it is not sealing correctly.

Eddiek

The plastic cap could have a crack that my not be noticeble.

feckin cheap plastic [:-]

Check Gas Cap comes on because evap system fails first test. Loose or missing gas caps #1 cause of ses light. Gas cap you can correct and save trip to dealer. There is a tool just for testing evap systems dealers and some repair shops will have it. You may not need gas cap. If condition keeps occurring a trip to repair shop may be required.