Nick,

Beginning with the '93 model year, 850s have a available under the hood a self diagnostic facility that allows you to read and input codes. This feature ended after the '96 model year and a scan tool is needed for newer models.This is nothing new to Volvo as other earlier models had something similar. To fully understand the diagnostic tool I highly recommend you purchase manual TP#3501201 (for complete explaination) and the "Pocket Data Booklet" from 1-800-25-VOLVO. This will be money well spent. To get you started...you will find two DLCs (Data Link Connectors) hanging off your ECM (Engine Control Box) next to the washer filler. The boxes labelled A & B provide you with a number of terminals that are dedicated to different functions of your car. You can both read codes (DTCs, Data Trouble Codes) or input codes to do things like re-setting your service lamps indicator or turning off the check engine lamp. Much has been written about this and you should be able to find plenty in the archives...

Good Luck and enjoy you new 850...