You didn't mention the year of your 850, but you have several options and some of them won't cost you a dime.

For early models, go to www.volvospeed.com and check out the instructions in "Bay 13." This is under the repair section and not only contains instructions for reseting the light, but how to read the codes. This works 93-95 850s.

If your 850 is 1995 or newer, you can reset the check engine indicator with an OBDII scanner. You can check to see if your's is OBDII compliant by looking for the sticker under the hood, or the connection on the center console. The conection is hidden under the coin holder labled OBDII. I have a 1995 850 and I use the Actron scanner that I purchased for $139 at a local auto parts store. It reads and clears codes on ANY OBDII compliant vehicle. This means I can use it on my wife's 2004 GMC as well as our 1996 Chrysler T&C.

Your other option is to visit your local auto parts store and ask them to read and clear the codes for you. Most of the national auto parts chains now offer this service at no charge. This can be helpful if you are dealing with a stray code that popped up once, but isn't an indicator of a real problem.