Daikin Packaged Unit Error Code U1

Full Diagnosis

U1 is a power-supply phase fault detected at the outdoor unit's main PCB (A1P) — either the incoming three-phase power is missing a phase (open phase) or the phases are connected in the wrong rotational order (reverse phase) — and the unit shuts down before energizing the compressor to prevent motor damage. This is a supply-side fault checked at power-up or restart, distinct from P1, which is an interphase voltage imbalance detected within the inverter circuit during operation, and from U2, which covers a momentary power interruption or abnormal voltage level rather than a phase-sequence or phase-loss problem.

All Recorded Causes

One incoming power phase missing (open phase) at the disconnect or terminal block. Two phases swapped, causing reverse rotation sequence. Loose, corroded, or burned power terminal connection. Blown fuse or tripped breaker on one leg of a three-phase feed. Defective outdoor unit main PCB (A1P) phase-detection circuit.

Verify Before Replacing Parts

1. Confirm the exact Daikin model family and packaged context before acting on a generic code description.
2. Confirm board LEDs, wiring polarity, and model compatibility before replacing control boards — this Daikin fault points at one incoming power phase missing (open phase) at the disconnect or terminal block.
3. Check for intermittent communication faults caused by connectors, moisture, or damaged harnesses, and rule out two phases swapped, causing reverse rotation sequence.
4. After the correction, run the system long enough to confirm the packaged fault does not return under normal load.