Code: UH🔧 Daikin Mini Split

Daikin Error Code UH: System Address Undefined (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 5 manufacturer & service sources

What the code means

Daikin's service manuals call UH "Malfunction of System, Refrigerant System Address Undefined" (VRV II manual Si39-302 and VRV III/RXYQ manuals) or "System Abnormality, Refrigerant System Address Undefined" (RXYQ-XA manual SiUS342003E). The system detects an indoor unit whose address was not set by the automatic address function, and the error is decided as soon as that's detected. Automatic addressing is the step where the outdoor unit assigns addresses to every indoor unit on its transmission line when power is first applied after installation or a wiring change, or when the RETURN (BS3) or RESET button on the outdoor board is held for 5 seconds. Daikin's simple self-diagnosis chart sums UH up as a malfunction of system with the same three causes. It is a wiring and addressing problem, not a refrigerant problem.

Likely causes

Safe DIY checks

⚠ Before you start

Turn off the breakers for the outdoor unit and every indoor unit before any inspection. Daikin's manuals warn to turn off the power switch before connecting or disconnecting connectors, or parts may be damaged. The outdoor board shares the electrical box with inverter capacitors that can hold high voltage after power is off, so leave wiring checks and the RETURN/RESET button to an HVAC technician.

  1. Think about what changed. UH most often appears after a new installation, added indoor units, rewiring, or a board replacement, so call whoever did that work first.
  2. Give it time after power-up. Daikin's flowchart waits 12 minutes from when power reaches the indoor and outdoor units before treating UH as a fault, and the system won't run during that time.
  3. Do a full power reset: switch off the breakers for the outdoor unit and all indoor units, wait a few minutes, switch them back on, and wait the full 12 minutes.
  4. Note whether UH shows on every indoor unit or just one. That tells the technician whether to look at the outdoor main board or a single indoor board.
  5. Don't open the outdoor unit to press the RETURN/RESET button or touch the transmission wiring yourself. The board sits next to high-voltage inverter parts.

🛠 When to call a pro

  • UH stays after 12 minutes and a power reset. A technician checks the indoor-outdoor and outdoor-outdoor transmission wiring against the installation manual.
  • The wiring was wrong. After fixing it, the technician holds the RETURN (BS3) or RESET button on the master outdoor unit board for 5 seconds to re-run automatic addressing, then waits up to 12 minutes.
  • The wiring is correct and UH shows on only some indoor units. Daikin's flowchart replaces the indoor unit PCB.
  • UH shows on all indoor units. Daikin's flowchart replaces the outdoor unit main PCB (A1P).
  • On some RXYQ systems, the newer manual also has the technician temporarily disconnect the outdoor-outdoor wiring to test one system at a time, and in some cases fit a DIII-NET extension adaptor.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Outdoor unit main control board (A1P)

Replaced when UH shows on every indoor unit and the wiring checks out; match the exact outdoor model number.

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Multimeter

Used by the technician to check the transmission wiring for opens, shorts and wrong connections.

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Transmission (communication) wiring

Wrong indoor-outdoor or outdoor-outdoor wiring is Daikin's first listed UH cause; follow the installation manual's wiring diagram.

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Sources

  1. Daikin RXYQ-XA service manual SiUS342003E, 3.73 System Abnormality, Refrigerant System Address Undefined (UH): detection, automatic address and RETURN (BS3) button, supposed causes, via ManualsLib
  2. Daikin RXYQ-XA service manual SiUS342003E, UH troubleshooting flowchart (12-minute wait, indoor vs outdoor PCB, DIII-NET extension adaptor), via ManualsLib
  3. Daikin VRV II service manual Si39-302 (RXYQ5~48M), 2.51 UH Malfunction of System, Refrigerant System Address Undefined, via mcgrp.ru
  4. Daikin VRV II RXYQ5MATL service manual, 3.52 UH Malfunction of System, Refrigerant System Address Undefined, via ManualsLib
  5. Daikin Industries, Simple Self-Diagnosis by Malfunction Code (UH malfunction of system: improper transmission wiring, defective indoor unit PCB, defective outdoor unit PCB)

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.