Daikin Mini-Split Code U4
Soft lockout
Signal transmission error between the indoor unit and the outdoor unit. The indoor unit either cannot receive data from the outdoor unit or the data it receives is corrupt.
Manual wording: “U4 Signal transmission error (between indoor unit and outdoor unit)”
Documented for: FTXS-L, FDXS-L, H-Series, FTXM-W/V pair systems; on RMXS, indoor unit to Branch Provider unit transmission
Likely causes, most common first
- Wiring error
- Breaking of the connection wires between the indoor and outdoor units (wire No. 3)
- Defective outdoor unit PCB
- Defective indoor unit PCB
- Disturbed power supply waveform
What you can safely check yourself
- Check the breaker or disconnect for the outdoor unit is on - the indoor unit will report U4 if the outdoor unit has no power
- Turn the system off at the breaker or disconnect switch, wait a minute, and turn it back on once
When to call a technician
U4 is the most common Daikin mini-split code and it is almost always the interconnecting wiring or a board. Checking wire No. 3 and the terminal board voltages requires a technician.
A Daikin mini-split throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Service Manual - Inverter Pair FTXS-L Series / FDXS-L Series (SiUS091133) (Daikin, FTXS09/12/15/18/24/30/36LVJU wall mount, FDXS09/12LVJU ducted, RKS/RXS09-36LVJU outdoor) · checked 2026-08-25
- Service Manual - Inverter Pair Wall Mounted Type H-Series (SiUS04-924_A) (Daikin, FTXS30/36HVJU indoor, RKS30/36HVJU and RXS30/36HVJU outdoor) · checked 2026-08-25
- Service Manual - Inverter Pair Wall Mounted Type FTXM-W/V Series (SiUS042113EA) (Daikin, FTXM-W / FTXM-V (Daikin Atmosphera) wall-mount heat pump) · checked 2026-08-25
- Service Manual - Multi-Split Type Air Conditioners RMXS-L Series (SiUS181631EA) (Daikin, RMXS48LVJU outdoor, BPMKS048A2U/049A3U branch provider, CTXG/CTXS/FTXS/CDXS/FDXS/FVXS/FFQ indoor units) · checked 2026-08-25