Unit Completely Inoperative

Question

Accepted Answer

Circuit breaker (NFB) tripped — overload or electrical surge. Check: Check dedicated AC breaker at distribution panel — verify if it is in trip position (middle). Measure primary voltage with multimeter (within ±10% of rated). What to do: 1) Identify trip cause (megger test compressor insulation, inspect wiring for shorts). 2) Resolve cause, then reset breaker OFF→ON. Never reset immediately — fire hazard if root cause unresolved. Faulty start capacitor. Check: Measure capacitance (μF) with LCR meter or multimeter in capacitor mode. Faulty if deviation exceeds ±6%. Visually check for swelling or electrolyte leakage. What to do: Replace with same rating (μF, voltage). Discharge residual voltage across terminals before replacement (short with insulated screwdriver). Classic symptom: humming sound with fan/compressor not spinning. Unstable power supply — low voltage or single phasing. Check: Measure each phase-to-phase voltage with multimeter. Faulty if deviation exceeds ±10% of rated or phase imbalance >3%. For single-phase units, check L-N voltage. What to do: Request electrical contractor to resolve supply voltage issue. Automatic voltage regulator (AVR) can be installed as temporary measure. Single phase detected: stop unit immediately — compressor winding burnout risk. Thermostat power failure or dead batteries. Check: Check if thermostat display is lit. For battery-powered units, measure battery voltage (AA ≥1.5V). For wired units, verify terminal supply voltage (typically 24VAC or 12VDC). What to do: Replace batteries (new AA ×2). For wired: inspect control circuit fuse and wiring. If display is normal but command not transmitted, re-tighten wiring terminals or replace thermostat. Safety interlock activated — low/high pressure cutout or overload trip. Check: Check controller error code. Verify low pressure switch terminal continuity with multimeter. Measure low-side pressure with gauge (R-410A cooling minimum 0.3 MPa required). What to do: 1) Low pressure cutout: refrigerant shortage → detect leak and recharge. 2) High pressure cutout: check outdoor unit heat rejection (clean coil) or refrigerant overcharge. 3) Overload trip: resolve cause, then manually reset. Never repeatedly reset without resolving root cause.

Unit Completely Inoperative

Likely causes

Circuit breaker (NFB) tripped — overload or electrical surge

Faulty start capacitor

Unstable power supply — low voltage or single phasing

Thermostat power failure or dead batteries

Safety interlock activated — low/high pressure cutout or overload trip

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