Update:
Installer called Mitsubishi and they recommended to replace a control board in the outside unit. He came by last week and changed it out. The fan was back to normal operation for two days. On the third day it started doing the same thing again. Installer is supposed to have another talk with Mitsubishi.

 

Interesting. I've never personally had this happen but I would look toward the high pressure switch as possibly causing this issue. The fan will modulate to maintain a head pressure, if the switch is bad then the fan will run erratically.

Tech support for Mitsubishi is pretty good. They should be able to point your contractor in the right direction.

 

Update:

Mitsubishi advised the tech to check the resistance on the thermistors in the outside unit. He came by yesterday to check and they were all within spec. Also, he tried to run the unit to see the problem happen with the outside fan. Unfortunately, I had shut the system off because of some other contractors doing work. When the tech turned it back on and raised the temp in cooling mode, the outside fan shut off. Damnit.

So, I left it running this morning and, sure enough, now the outside fan stays on when the compressor shuts off. So it looks like it needs to run for a while before the malfunction with the outside fan happens. I raised the set temp to 86F inside in cooling mode and the outside fan is still cranking away after several hours, compressor is off.

I called the installer's office and talked with the tech again. He says there will be another call to Mitsubishi to see what they want to do next. I suggested that he look at the pressure sensors as there may be a problem with that component. He said the fan runs on temperature, not pressure. No idea if that's right or wrong as I'm not a Mitsubishi tech. Hopefully this latest episode will produce some results.

 

Final update:

All the running around was due to bad information from Mitsubishi tech support on the initial calls. On the controller for this unit, it is programmed to function in "low ambient mode" when set to cooling mode and the outdoor temp is 67F or below. During "low ambient" operation the outdoor fan will be energized continuously to move air over the thermistors to sense the proper air temperature. I got my hands on a copy of the technical manual for the controller board and indeed there is a section in it that explains this behavior.

It would be helpful if this information was included in the regular owner's manual. Also, the tech support operators at Mitsubishi have no idea about this at all. My installer and I both had to persist and demand to speak to upper level tech support in order to get a proper answer on this. What a waste of time for everyone.

Thanks for the comments and suggestions provided earlier. I did relay those points to my installer.

 

Never heard of such a thing but thanks for the followup. You may have saved a tech out there from spending a lot of time when he runs across it in the future.