A while ago I had my spark plugs and wires changed and since then I haven't been able to use my 850's stereo. I think at one point it asked for a code but I'm not sure at the time my wife was driving it and I was driving another car. Now whenever I turn it on the display just says "OFF". How can I resolve this? Or does it have to be taken to a dealer?

 

Please use the search function. There is a lot of information for this. You need a code when you disconnect the battery to be able to use the radio. If you attempt to turn it on and enter in the wrong code too many times, it locks itself out. You have to put the key in the ignition in spot I and let it sit for approx 2 hours, and then you can try the code again. If you don't have the code, the dealer might be able to look it up via your VIN number.

With a simple search:

https://volvoforums.com/forum/volvo-...t-issue-18975/

http://www.matthewsvolvosite.com/200...des-explained/

http://www.matthewsvolvosite.com/for...hp?f=1&t=12188

 

the code is often written somewhere on the stereo, pull it out and check

 

I always suggest that people write it inside the passenger side sun visor.