LG Heat Pump Error Code P4

P4 on an LG heat pump signals a reverse phase or open phase error on the 3-phase power supply. The unit expects power on lines L1(R), L2(S), and L3(T) in a specific order and at the correct voltage of AC 380V. If the phase order is reversed or one phase is missing, the heat pump shuts down to prevent compressor damage and displays P4.

No. The P4 fault causes the unit to lock out, and you should not attempt to override or bypass this protection. Operating a heat pump compressor on reversed or missing phase can destroy the compressor motor in a very short time. Leave the unit off until the power supply issue is correctly diagnosed and repaired.

If the fix is simply swapping two supply wires to correct the phase sequence, labor costs from an HVAC technician or electrician are typically in the range of $75 to $200 depending on location and service rates. If the fault is caused by a failed contactor or a blown fuse, parts and labor together generally run $150 to $400. A utility-side problem would need to be addressed by the utility provider at no parts cost to you, though a service call from an electrician to confirm the diagnosis may still apply.

Start by measuring voltage at the CN77 cable across all three wire pairs: red to white, white to black, and red to black. Each pair should read AC 380V. If voltages are normal, the phase sequence is reversed and you need to swap any two of the incoming 3-phase supply wires, then press the reset key (K3). If any voltage reading is abnormal, trace the missing phase back to the breaker, fuse, contactor, or utility supply to find the open circuit.

Pressing the reset key (K3) is part of the repair procedure, but only after the underlying wiring issue has been corrected. If the phase sequence is still reversed or a phase is still missing when reset is pressed, the P4 fault will return immediately. The reset is meant to be used after the incoming power wiring has been verified or corrected.