The way I understand it, the Check Engine light comes on to get you to go to the dealer. This is similar to the Service light. The Service light on my 1990 760 comes on every 5000 miles, while the Service light AND Check Engine light on my 1996 850 come on every 10000 miles, according to a dealer I spoke with. There maybe nothing wrong whatsoever, but Volvo wants servicing and repair work to be done by Volvo. The only way to turn the light off is to go to the dealer, where they will charge you a "diagnostic fee" of around $70. That's why the Service and Check Engine lights in my Volvos remain on all the time.