If your 850 is a '95 or older you can check the codes using the code boxes under the hood next to the windshield washer fill tube. Take the top off the box marked A. Unclip the test lead and put it in the socket marked '1'. Turn the ignition on (don't start the engine, just have all the idiot lights on in the dash). Push the button on module A for about 1 sec. The LED will flash a series of codes in groups of 3. 1-1-1 indicates no codes stored. After each set of 3, press the button again for 1 sec. Repeat until the first code repeats. Copy all the codes down as they appear, then go to Bay 13 at Volvospeed.com and you can find a list of what the A1 codes (transmission) mean.

Good Luck!