I found out today that Volvo's VIDA/DICE software has a very helpful cooling system troubleshooting diagnostic procedure. It helped me determine that my fan was working properly (after cleaning and reconnecting the fan and ECT connections).
Of course, it's best to first look for DTC codes that might point to any problem. With or without codes, also try the above VIDA diagnostic located here: Information Tab -> Fault Tracing -> Symptom-Related Diagnostic Proceedures -> Engine with mounting and equipment -> 28 Ignition and control systems -> 284 control sytems, fuel supply -> Checking Engine Fan & Cooling System.
Once there, follow the guided diagnostic to troubleshoot your cooling system. At one point, you can click on the VCT2000 icon. VIDA will then show you the current engine temp and allow you to manually run the fan on 6 different levels. Basically, if the fan is working well on all speeds during this test, and your system is full of coolant, then your problem is likely a stuck closed thermostat or a faulty ECT sensor (sometimes called ETC).
A simple test for the thermostat is to let the engine come up to operating temp, then feel the thermostat housing on the engine, then feel the upper radiator hose attached to it. If the hose is still cool, then the t-stat is likely stuck closed. If the t-stat is OK, try swapping out the ECT sensor.
Of course, overheating can be caused by more serious issues such as a clogged radiator or a blown headgasket, but those problems tend to have other symptoms. Also, always check that your coolant is at least a 50% mix.
BTW, there's a bunch of other diagnostic procedures at this location in VIDA as shown in the attached photo. I'm sure that I'll be checking them out sometime
I hope this helps someone cool off their Volvo!
