Daikin U2

What does Daikin U2 mean?

The U2 code means the inverter has detected a supply-voltage fault — under-voltage, over-voltage, phase loss on three-phase equipment, or a DC-bus reading that does not match the expected AC input. The PCB trips to protect the IGBTs and electrolytic capacitors from out-of-spec conditions. Root causes split between real supply problems on the building side and faults on the voltage-sensing circuit of the PCB itself.

Symptoms

• Unit will not start and shows U2 at power-up
• Intermittent U2 trips during the day, often matching building load events
• Outdoor fan spins slowly or not at all on start
• Compressor stops under heavy load with U2
• Indoor unit displays fault immediately after a grid disturbance

Common causes

• Under-voltage or over-voltage at the outdoor disconnect
• Phase loss or imbalance on three-phase equipment
• Loose main feed or disconnect-switch terminal
• Voltage-dip or surge event damaging the PCB sensing circuit
• Defective outdoor unit PCB voltage-sensor circuit

Diagnostic steps

1. 01

   Measure supply voltage at the outdoor disconnect

   Check line-to-line and line-to-neutral at the disconnect. Confirm within nameplate tolerance under both idle and run conditions.

2. 02

   Verify phase balance (three-phase units)

   Measure each phase-to-phase combination. Imbalance over 2% warrants investigation at the supply side.

3. 03

   Check for phase loss

   With all phases expected, a missing leg will trigger U2 on start. Confirm the upstream breaker and contactor are intact.

4. 04

   Tighten feed terminals

   Isolate power, then torque-check the feed lugs at the disconnect and on the outdoor terminal block. Loose connections cause voltage dips under load.

5. 05

   Check neutral and ground

   Confirm solid neutral and a low-impedance ground back to the building earth. Floating neutrals produce erratic voltage readings.

6. 06

   Replace the outdoor PCB

   If the supply is verified clean and stable, the onboard voltage sensor is suspect — swap the outdoor PCB and retest.

When to call a professional

Call a licensed electrician if supply voltage reads out of tolerance or unstable, because root causes often sit upstream of the HVAC equipment — transformer sizing, feeder length, or a shared load problem. An HVAC technician should handle outdoor PCB replacement, because matching part numbers, setting capacity DIPs, and verifying inverter DC-bus voltage on restart require manufacturer training. Do not repeatedly reset U2 on unstable supply — each trip cycle stresses the capacitor bank.