Try DIY first — may need a proVerified by FixOffice
Daikin U4 — Indoor/outdoor transmission (communication) error
Applies to: Daikin mini-split and light-commercial systems
What this error means
Communication between the indoor and outdoor units failed — Daikin's equivalent of Mitsubishi's E6. Wiring faults, a power interruption to one unit, or board failure.
Symptoms
- U4 on the controller or via the remote's fault-check
- System stops or one unit dead
Try this first (safe DIY steps)
- Full power cycle at the breaker for 3–5 minutes (both indoor and outdoor circuits)
- Confirm the outdoor unit has power (disconnect switch, breaker)
- Note recurrence pattern for the technician
Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.
When to call a technician
On recurrence — inter-unit wiring and PCB diagnosis.