We have two Mitsubishi A/Cs: 9k BTU for down bedroom and 18K BTU for up area. The bedroom unit is used every day while the up A/C rarely used as no one living there. This morning, my wife noticed that the 18K outside unit was running albeit very quiet and apparently just fan. After checking the inside unit was off so I turned it on and it started pumping out cold air immediately. I then turned it off and the inside unit "shut down" but the outside unit still running. The 9K unit has been off for several hours.

I shut off the breaker and it stopped running. Turned on the breaker and it started up again.

Both units are on the same circuit (10mm2/32a) and, to our knowledge, this has never happened previously.

Any ideas on what might be happening before I call out the service guy?

My thought that it's just running a fan only must not be correct as it's drawing 5 amps. It appears that it somehow turned itself on and won't shut off. The external power connections look nice and clean so must be something gone wrong inside. Jeez.

Believe there is a time delay relay (to prevent compressor from operating too soon after inside unit turn off or power failure) and these can fail in on position - time for a visit I suspect.

Believe there is a time delay relay (to prevent compressor from operating too soon after inside unit turn off or power failure) and these can fail in on position - time for a visit I suspect.

The last time that unit was turned on (except for today's testing) was last July. I'm guessing ants or gecko somehow turned it on. It's not RCD protected so don't know if a fault. The ground wire connected to the frame is placed in a hole drilled into the concrete even though a proper ground is included with the power feed 2 meters away. Nothing from a touch though.

So the service guys came and after fiddling around for 30 minutes found a relay (not covered by warrantee, of course) frozen open or closed not sure. B1500. Grrrr.

Believe there is a time delay relay (to prevent compressor from operating too soon after inside unit turn off or power failure) and these can fail in on position - time for a visit I suspect.

The last time that unit was turned on (except for today's testing) was last July. I'm guessing ants or gecko somehow turned it on. It's not RCD protected so don't know if a fault. The ground wire connected to the frame is placed in a hole drilled into the concrete even though a proper ground is included with the power feed 2 meters away. Nothing from a touch though.

If covered by warranty it wouldn't have failed ☺