Previous Owner (that I am aware of) :
Control Arms and Struts/Shocks probably also
I think the O2 sensors.
Since my ownership:
Selling dealer replaced failed Strut Mount before we bought it, which was causing a loose clunking, although vehicle still felt pretty solid otherwise.
Running a bit rough, where fuel economy wasn't that good when purchased. Did OE plugs, wires, cap, rotor, cleaned PCV system, replaced fuel filter, gas cap, all cracked/brittle vacuum hoses. Replaced split elbow at Fuel Pressure regulator. Ran some FI cleaner though a couple times.
Seatback Cable (common problem), driver side power seat wouldn't recline was completely inop. Paid $275 for dealer to replace.
Wife was going around a corner and it suddenly died on her. She pulled over, let sit for about 30 seconds, fired right back up and was able to bring it back home for me to look at. I ended up just replacing the Fuel Pump and Fuel Injection relays, simple and fairly inexpensive. Never had this happen again.
Cooling fan relay would stay on after shutting off and would not shut off. Replaced relay, was easy to to do, relay cost $79.
Needed to replace upper torque rod mount, side engine mount and lower tranny torque rod mount.
Water leaked into cabin due to cowl drains being blocked down inside the rubber hoses.
Exhaust leak, caused by the three nuts at the downpipe flange needing to be tightened. Pipe also from cat to muffler shifted slightly, had to pull clamp, realign then tighten down really good to fix this leak.
Shifter bulb, Climate Control Bulb out, bulbs in most switches out (unfortunately you have to replace the entire switches so I just don't worry about it).
Interior squeaks, creaks and rattles solved by thoroughly lubricating all the door latches and catches with Permatex Dielectric Grease. Lubed tailgate latch and catch, replaced and lubed rubber stops, replaced tailgate struts. Lubed rear seat latches and catches. Lubed glovebox side hinges, top lip, latch and catch. Pulled passenger side speaker panel, lubed plastic that makes contact with the dash underneath.
Currently one of the front Wheel Bearings is making some light noise you can hear when coasting. Will likely need to replace soon, not loose or having excess play yet though.
One of the dual climate control temp sensors on one side started making a very annoying rattling type noise. At first I thought it was an interior panel until I figured out there was a little fan inside the temp sensor that was making noise. Paid like $80 for a replacement.
Slight lifter tapping sometimes after starting up when cold after not being driven for a day or so. Usually only lasts for maybe half a minute, Synthetic 10w40 helps along with a couple engine flushes. Plan on doing another flush and couple more premature oil changes, should cure it.
Seat heaters get too hot even after recall had been performed. Dealer said it is normal, I'm thinking temp sensor have slipped out of the cusions or gone bad though. I just don't leave them on for long.
Great vehicle, understandable problems with this mileage. I understand no vehicle is trouble free let alone a 13 year old vehicle with over 150K on it.
Very solid, pretty fast ride with pretty good fuel economy once all the maintenance related stuff mentioned above was taken care of.
Sporty handling with good balance of pretty good ride confort.
Nice and roomy interior for not being a very large vehicle. Feels much larger than the newer 960's do, which for some reason feel cramped.
Volvo's 5cyl engine sounds really nice.
The squeaks, creaks and rattles were annoying until fixed by simply greasing things up with dielectrid grease as mentioned above.
The fact that you have to replace and entire switch when a bulb goes out wasn't very forward thinking.
Otherwise great, sold vehicle you can rely on.
Way more sophisticated, safe and far better quality even than many newer or even brand new American vehicles. Too bad newer Volvos weren't still made the same.
COMMON OVERLOOKED 850 PROBLEMS:
I hear it is very common for the Cooling Fan Relays to fail in these vehicles, often this will cause intermittent cooling fan operation or it won't function at all. Many people don't even notice before it's too late so I'd highly recommend just replacing it as preventative maintenance.
I also have heard from a couple Volvo techs about driveability problems caused by use of non Volvo OE Spark Plus, especially BOSCH. I've heard that many aftermarket plugs just don't work right.
Another common problem as mileage gets up there is the Fuel Pump and Fuel Injector Relay, which causes random cutting out. Simply replace them preventatively to prevent this.
The 3 turbo downpipe nuts at the flange often work their way loose causing exhaust leaks. If you have an exhaust leak, check this. Make sure you also check the downpipe bracket bolt that tends to work its way loose over the years.
Clean the PCV system to prevent excess crankcase pressure, which will cause or contribute to oil leaks.
Replace cracking or brittle vacuum lines and rubber elbows, especially the rubber vacuum lines around the turbo.
MY THOUGHTS ON 850 TRANSMISSIONS:
I have a feeling these transmissions are fine if not abused and the fluid is regularly changed.
I think the number one problems is lack of regular fluid changes (drain and refill 3.5 quarts)
I believe flushes actually contribute to killing them because the fluid isn't replaced as fast as it is drained when flushing. I don't think it has to do with metal fragments being loosened up as a lot of people say.
Another common transmission killer is simply overheating from bad cooling fan relays and bad thermostats people often have no idea about or neglect.
I also think sometimes people get ripped off where for example it's actually an electronics fault like a bad PNP switch, shifter microswitch or something.
MY THOUGHTS ON BUYING AN 850:
The mistake most people make is buying an older used Volvo and thinking "it's a Volvo" and that they won't have to service it or make any repairs.
If you don't know much about Volvos and aren't interested in learning, another common mistake people make is not finding the right people to service it. People you can trust and will properly and thoroughly maintain it.
The last mistake people make is buying a Volvo that wasn't taken care of properly and trashed. When I went to look at 850's a lot of them looked like they had been neglected and abused, just by the kind of wear and common lack of any service paperwork from many people selling them.