Daikin Mini-Split Code U9
Soft lockout
Signal transmission error between an indoor unit and the outdoor unit within the same multi-zone system. One zone's indoor unit or its expansion valve is faulting and taking the whole system's communication down.
Manual wording: “U9 Signal transmission error between indoor unit and outdoor unit in the same system”
Documented for: RMXS and other multi-zone systems
Likely causes, most common first
- Improper connection of transmission wiring between indoor and outdoor unit
- Defective indoor unit PCB in another zone
- Defective electronic expansion valve in an indoor unit of another zone
What you can safely check yourself
- Turn every indoor unit on and note which zone shows the fault
- Turn the system off at the breaker or disconnect switch, wait a minute, and turn it back on once
- Wait at least 2 minutes before reading the code again - the manual says a fresh U9 needs 2 minutes to settle
When to call a technician
U9 points to a different zone than the one showing the code. Tracing which indoor unit is at fault across a multi-zone system is a technician's job.
A Daikin mini-split throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- 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