To start, the Ignition key must be in position II, engine not running. Locate the two diagnostic sockets just inboard of the washer fill cap. The "A" connector (shown above) has a jumper that is stored in the lid of the "A" diagnostic connector. The jumper is used for both diagnostic connectors. Listed below is the information retrievable from the various connections. A1 Transmission B1 Climate Control A2 Fuel System B2 Cruise Control A3 ABS B5 SRS A5 TCU (Transmission Control) B6 Power Seat A6 Ignition System A7 Combined Instrument Normally "Check Engine Light" information will be found in A2. Put the jumper into A2 and push the black button for one second. Count the red LED light as it blinks back information, blink, blink, pause, blink, blink, blink, pause, blink, blink, is an example of the response you would receive. 2-3-2 would be the code retrieved, now push the button again for one second, again count the blinks, keep checking for codes until one of them repeats, this means you have read all the codes. Now to clear the codes, push and hold the button for 6-7 seconds, release and as soon as the LED comes on push and hold the button for another 6-7 seconds. Now push the button for one second and read the codes again, you should get 1-1-1 (no faults). You Must Read All Codes before Clearing. Turn the ignition off and then start the car, the check engine light should be out. It is always a good idea to check for codes, clear them and then drive the vehicle to see if they return before replacing parts. The transmission can ask the check engine light to be turned on, so if you try clearing A2 codes and they wont go away try reading the transmission (A1), clear them , and then go back to A2.
After contacting a Volvo dealer, they recommended the Haynes series of manuals for the 850, each section details the codes and faults detected.