CHECK ENGINE LIGHT STAYS ON on 2002 GMC Envoy

THE CHECK ENGINE LIGHT HAS BEEN ON FOR SEVERAL WEEKS NOW. I NOTICE NO OTHER PROBLEMS WITH THE VEHICLE. MAYBE IT IS MY IMAGINATION, BUT IT SEEMS LIKE MY AVERAGE MILES PER GALLON HAS GONE UP SINCE THE LIGHT CAME ON.

3 answers 2 comments
The check engine light means that there's a failure somewhere in your emissions control system. RepairPal has some info on it: http://repairpal.com/check-engine-light

Could be a number of things, and problems in emissions can change mileage. Normally, fuel economy gets worse, though. Best to have it diagnosed and fixed soon to avoid additional problems or damaged components.
While this is general good advice, my experience is that it is a faulty gas cap gasket. I have had good reponse from both factory and after market replacements. But, the problem does come back eventually. Better if there were a pack of gaskets available.
That is correct what the master said. I had that very same problem and ignored it for just about a year. Now I'm having problems. Milage gradually worsened, had to replace something that kept the engine reved up when I turned on the car, and just recenty changed the water pump. Taking it in tomorrow to fix problem w/power loss and vibration. The car is a 2002 w/ 135,000 miles. No major issues until now :-(
I've had this problem, had mechanics do a full inspection, and found nothing. When I had an emmission check, the mechanic indicated I had a seal problem with the gas cap - which would cause the check engine light. I have no other issues for years with the car...so I've concluded my gas cap on/off is the issue
I had this problem a few years ago. Eventually i found out i had a gas tank leak.