Daikin U4 fault code

What it means

Indoor/outdoor transmission error

Transmission between indoor and outdoor unit has failed (Daikin also lists outdoor power off and address mismatch).

Safety: Safe isolation before opening terminal covers — mains present on the interconnect.

UK models this code applies to

From the Daikin service literature we hold (Daikin CTXM/FTXM-M operation manual 3PEN393186-10J, malfunction-code table (remote self-diagnosis)) — not a scraped list.

Now the harder half: proving what caused it

U4 names a symptom, not a root cause. In the app, the same code opens a guided investigation: what to measure first, the expected values for your exact model, and an honest verdict — including “nothing in this machine is broken” when that's what the readings say.