2010 Volvo XC60 - Runs hot but fan stays off

My Volvo has a quirk. It will tell the engine is running hot. But the engine fan will not run.

If I turn off the ac and the ac fan, the engine fan will run and cool the engine.

Any clue what is wrong. My mechanic (excellent, respected in community) spent two hours hunting for the cause.

your vehicle has 2 fans, 1 for the engine which is working, and 1 when the a/c is turned on, that is not working. the fan for your a/c either has a bad relay or is faulty and needs to be replaced.