Safe checks first
- Create real demand. Set cooling 5+ degrees below (or heating 5+ above) room temperature and give it ten full minutes. Inverter startup is deliberately slow and quiet - listen for the soft ramp rather than a bang.
- Check the indoor display for a code. Communication errors (EL 01 class on many units) and protection codes explain an idle outdoor unit precisely - look yours up in the library before anything else.
- Breakers and disconnect. The outdoor unit typically has its own breaker AND a disconnect box on the wall beside it. Confirm both are fully on (a half-tripped breaker looks on; cycle it fully off then on). If a breaker re-trips, stop - that's a technician call, not a retry loop.
- Weather states. In heating, steam and a stopped fan = defrost (normal, minutes). In severe cold below the unit's rated floor, protection idling is the spec working. In cooling on a cold day, many units refuse low-ambient cooling by design.
- Power cycle everything - breaker off five minutes, on, then demand again.
Beyond the safe checks (technician scope)
Failed outdoor fan motor or its drive board, compressor start failures, control-board or communication faults (the wire run between units), protection trips from voltage or pressure. All of these are diagnosis- with-meters territory.