Daikin error code U4
What it means
Transmission failure between indoor and outdoor units.
Possible causes
Communication wiring open, short or reversed polarity Incorrect address or system configuration Failed communication interface on PCB
Suggested fix
Confirm continuity and polarity of the communication bus and recheck address and setting codes; once wiring and configuration are verified, test and replace the defective indoor or outdoor PCB communication stage as needed.
Safety first. Isolate the power supply before inspecting any unit. Refrigerant-circuit
and electrical repairs must be carried out by a qualified, registered technician.
Other Daikin codes
- A1 — Indoor PCB defect or internal failure.
- A5 — Freeze-up protection activated for the indoor heat exchanger.
- A6 — Indoor fan motor fault.
- C4 — Faulty indoor heat exchanger temperature sensor.
- C9 — Faulty indoor suction air (room) temperature sensor.
- E1 — Outdoor circuit board fault.
- E7 — Outdoor DC fan motor fault.
- F6 — High pressure control activated in cooling mode.
- U0 — Refrigerant shortage or undercharge detected.
- U2 — Power supply voltage drop or main circuit overvoltage.