LG Therma V fault U3

What it means

U3 indicates an abnormality associated with the Reactor PCB assembly / IGBTM (IGBT module) on three-phase 12, 14 and 16 kW units. Diagnosis is performed by checking diode-mode voltage drops across the IPM P, U, V, W, R, S, T and N terminals to determine if the IGBTM/PCB has failed.

Likely causes

1. Reactor PCB assembly fault
2. IPM (IGBT) internal failure
3. IPM/IGBTM failure
4. Internal rectifier diode failure
5. Damaged power transistor within module
6. IPM damage (short or open between P and N terminals)
7. PCB damage (diode readings differ from expected table)
8. IPM failure
9. IGBT module internal short or open circuit
10. IGBTM (IGBT module) internal failure
11. Fan locked
12. Obstruction/shield around the heat exchanger
13. Insufficient distance between product and wall
14. Piping condition issues such as pipe bending
15. EEV operation/connection status fault
16. Abnormal refrigerant pressure
17. Compressor restraint
18. Compressor wiring reverse phase loss
19. Short circuit or opening of internal connection line in compressor
20. Compressor winding resistance out of spec
21. Compressor insulation resistance failure
22. Inverter PCB assembly or connector connection fault

Where to start

1. Remove PCB connector after power off (PCBA only measurement)
2. Set multimeter to diode measurement mode
3. Measure P ~ U, P ~ V, P ~ W, P ~ R, P ~ S, P ~ T with black probe on P and red probe on respective terminal