Air conditioner [Outdoor unit not running]what to check

An indoor unit that blows air while the outdoor unit sits silent is a specific and quite readable symptom, because the outdoor unit is where the cooling actually happens. The indoor fan will run happily on its own and move room-temperature air around, which is why this presents as an air conditioner that is on but useless rather than one that is dead. Start with the idea most people bring to it, because it is wrong for this equipment. A domestic split has one mains supply feeding both units. Daikin’s installer guide for their outdoor units warns not to connect the power supply to the indoor unit at all; Mitsubishi’s service manual for the MSZ wall units has the main supply landing on the outdoor unit’s terminals, with the indoor unit fed over the three-core connecting wire; Toshiba wire the power cord to the outdoor unit as standard, with a supply into the indoor unit as the option. Whichever end it arrives at, the other end is fed from it.

So if the indoor unit is lit and taking commands, the outdoor unit has power, and an isolator or breaker that had been switched off would have taken both units down together. That does not make the isolator irrelevant — a system that is completely dead, with no lights and no response to the remote, is exactly the case for the isolator and the consumer unit, and it is the first thing on Daikin’s and Toshiba’s owner checklists for a unit that does not operate. It just means that a live indoor unit and a silent outdoor unit is not a power problem, and the explanations are elsewhere. Most of them are not faults at all. Compressors do not restart instantly: Toshiba publish a three-minute protection feature that stops the unit being activated for three minutes after a sudden restart, and Daikin say the system resumes about three minutes after a power failure or after stopping itself on a large voltage fluctuation, so a unit switched off and straight back on sits doing nothing for several minutes by design.

In heating, a reversible unit periodically melts frost off the outdoor coil: Daikin say hot air stops coming from the indoor unit during defrosting and that the frost melts into water or steam at the outdoor unit, and Toshiba say the fans stop during defrost operation. And a system that has satisfied the room temperature has nothing to do, so the outdoor unit stops while the indoor fan carries on — Toshiba describe exactly that, with the fan speed reduced and the outdoor unit stopping once the set temperature is reached. None of those are faults. Beyond that, the published faults in this area are mostly communication and power. Manufacturers publish codes for transmission failure between the indoor and outdoor units, and the causes are wiring, the power-up sequence and the supply rather than the machinery. Mitsubishi make a specific point that is easy to miss: they name the system having been powered up incorrectly as a cause, and their remedy is to repower in the correct sequence, indoor unit first and then outdoor. That is an installer action, but it explains why a fault sometimes appears immediately after unrelated electrical work.

1. 1

   Look at the indoor unit first. If it is lit and answers the remote, the supply is on and the outdoor unit has power — skip the isolator and go to the next steps. If nothing at all is lit, check the isolator near the outdoor unit and the consumer unit, which is Daikin’s and Toshiba’s first owner check for a unit that does not operate.

   If a breaker has tripped, reset it once. If it trips again, leave it off and get it looked at — a protective device operating repeatedly is detecting something rather than being temperamental.
2. 2

   Wait at least three minutes before concluding anything, and preferably ten. Toshiba and Daikin both publish a delay of about three minutes before the unit runs again after a restart or a power cut, and that delay is deliberate.

3. 3

   Check the setpoint against the room. A system that has reached temperature stops the outdoor unit and keeps the indoor fan turning, which is the commonest reason of all for a quiet outdoor unit.

4. 4

   If you are running in heating and the unit is steaming with the fan stopped, leave it alone. That is a defrost cycle and it ends by itself.

5. 5

   Look around the outdoor unit. Nothing should be blocking its air inlet or outlet — planting, a cover, fencing or packed leaves — because a unit that cannot reject heat will trip its protections and stop. Clear it without removing any panels.

6. 6

   If there is power, no delay, and no defrost, retrieve the fault code and pass it on. Communication faults and compressor faults look identical from the garden and are very different jobs.