My S80 used to do that every few months.... disconnect the battery for 30 minutes, or conversely disconnect the negative and touch it to the positive to discharge. Reconnect and all is back to normal. Mine also sometimes likes to put the fuel gage and/or the temperature gage to zero while this occurs.

 

The fan module is going out. Replace the entire assembly before it stops working and you overheat the engine.

 

Second that. That's exactly what happened in mine. At first it stayed on all the time, draining the battery, and it died completely shortly afterwards. When the fan module is about to go out, the fan motor isn't far behind so the best option is to replace everything at once.

 

I had same symptoms but it happened to be the AC refrigerant, low. After recharge, all was well.


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Similar to what Oliovlov mentions, found this is on the VolvoXc site: According to Volvo mechanic, when the pressure in the aircon gas goes below a certain PSI the radiator fan switches on by direction of the high pressure sensor. Pressure was very low so I did add Freon and that seems to have fixed the issue, but have also purchased a complete assembly from Erie as suggested by hq_ and Ihatespeedbumps.

Thanks for the help on this.