Daikin U4 Error Code: Signal error between indoor and outdoor unit
Updated October 2026 · Sourced from Daikin documentation
The data sent from the outdoor unit cannot be received normally by the indoor unit, or the content of the data is abnormal. The manual lists wiring error, broken connection wires, defective boards or a disturbed power supply waveform.
Is it safe to keep running? Check the steps below first. Wiring and board checks between the indoor and outdoor units must be done by a licensed technician.
U4 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| U4 Shown on the remote controller (and as blinking lamps on the indoor unit) | Signal error between indoor and outdoor unit | Daikin FTXS-L / FDXS-L inverter pair ductless series (09 to 36 class, models FTXS09-36LVJU, FDXS09-12LVJU) |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Switch the unit off at the breaker, wait a minute, then switch it back on and see whether the code returns.
🛑 Stop and call a licensed pro if
- Wiring and board checks between the indoor and outdoor units must be done by a licensed technician.
- Any code pointing to refrigerant, the compressor, high pressure or the outdoor unit board. Refrigerant work legally needs a certified technician.
- Burning smell, sparks, or a breaker that trips again after a reset.
- Water leaking from the indoor unit that does not stop after the drain is clear.
Safe first steps for any Daikin code
- Write down the exact code and how it shows (remote display or number of lamp blinks). Similar codes mean different things on different series.
- Turn the system off at the remote, switch off the breaker or disconnect for the unit, wait a few minutes, then turn it back on.
- Clean or replace the indoor unit air filters, and make sure nothing blocks the indoor or outdoor unit.
- Check that the condensate drain line is not blocked or kinked where you can see it.
- Replace the remote controller batteries if the display is dim or the unit does not respond.
Source: Daikin Service Manual SiUS091133 - FTXS-L / FDXS-L Series, Troubleshooting p.109. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
Shaun Cain · Certified HVAC & Refrigeration Technician · Ask an HVAC technician