Air Conditioning Cuts Out, Sometimes

My a/c has been cutting out (warm air coming through) intermittently. If I turn it off for a few minutes, it generally comes through nice and cold again; sometimes the cold lasts for a while and sometimes not. It can happen after just 10 minutes of driving 45 mph, or after 1 hour of driving 70 mph.
My mechanic drained the system and re-charged it, but that hasn’t helped - although he is a terrific diagnostic mechanic, he is puzzled.
Any ideas?

Year and trim on your Subaru Forester? My best guess, if your mechanic checked your A/C for leaks and filled it to the correct pressure, is the expansion valve. It’s stuck open.

If it is blowing warm air, and you can feel air coming out the vents, then it is not icing up. Todays AC systems are turned on and off by the computer, the switch(es) on the dashboard are just one input. Other inputs include throttle position, manifold vacuum and various pressure and temperature sensors. If an engine coolant temperature (ECT) is sending the wrong temperature to the computer, it might think the engine is overheating and turn off the AC, that just one example.

It could also be that one of your engine cooling fans is working intermittently or simply that the compressor clutch relay is going bad. When it is blowing warm air, pull over if you can and raise the hood and check if the engine cooling fans are running. also when it is working, look at the front of the compressor at the pulley the belt goes around. Have someone turn the AC on and off a few times until you can tell when the clutch is engaged and the compressor running. Then when it starts blowing warm air, you can check to see if the clutch is engaged or not. This information will help your mechanic find the root cause.

Thanks, Keith - I’ll have to get some help to know where to look, so I’ll swing by to ask my mechanic to show me in the next day or two. Thanks again!