Two items. The thermostat is located behind the power steering pump, next to the intake manifold where the top radiator hose enters the engine. Two torx screws hold it down. If the OEM thermostat fails, it normally does so by failing to close. An open thermostat will keep the engine temp cold for a very long time after start up, most notible in the winter when you want cabin heat.

There is also the engine coolant temperature sensor, just below the thermostat, with a 2 wire plug. If the ECT sensor fails, there is usually a code stored. The ECT sensor tells the ECU when to turn on the engine fan.

If your 850 is a 1994-5, the DTCs are scanned from the engine bay. 1996+ cars need a scanner, but some scanners are better than others. Usually, those that can read Ford engines work well with Volvo, but only do engine related scans.

Go to an autoparts store and borrow a scanner, the check engine light does not need to be on for codes to be stored. A very good Volvo indie will have a Volvo scan tool, which can check the fan operation, just like DIY for 94-5 cars.

Klaus
--
Proud owner of a 1967 220S