Unfortunately, you would have to retrieve the code to find out what caused the Check Engine light to go On. You need a scan tool compatible with OBD-II. If you go to a Volvo dealer, they will probably charge you at least 0.5 hrs of labor for the simple code retrieval. Alternatively, you can invest in a simple OBD-II scan tool (> $100) and do it yourself. (Sooner or later you will need this tool anyway if you are planning to do any maintenance on your car.) Or, you can try borrowing the scan tool from an auto parts store (AutoZone, Trak Auto, etc.) Good luck and keep an eye on the cooling system. Check the oil cooler for leaks and front control arm bushings for wear. You can also start saving money for the PNP switch replacement.

Raphael