Daikin AC Error U4 — Indoor-Outdoor Communication Fault

What it means

Daikin air conditioner U4 is the indoor-outdoor communication error documented on Daikin Global's official error codes page.
Daikin's published explanation: 'communication error between indoor and outdoor units — the wires between the indoor and outdoor units are not connected properly.'
It means the indoor unit and outdoor condenser can't exchange data over the F1-F2 communication wires.
Most commonly the cause is physical wiring (loose, damaged, or installed incorrectly), not a board failure.

Affected Models

Daikin split-system residential air conditioners (FTX, RX, FTKM, RKM series)
Daikin Sky Air commercial split systems
Daikin VRV systems (U4 appears between indoor unit and BS box or outdoor)
Daikin ducted systems with separate indoor air handler and outdoor condenser
U4 is documented on Daikin's master error code list as a communication-bus fault

Common Causes

F1-F2 communication wires loose at indoor or outdoor terminal block
F1-F2 reversed (polarity matters on Daikin — F1 to F1, F2 to F2)
Communication wire damaged by rodents or weather (most common on outdoor sections)
Indoor or outdoor unit lost power to the control board
Indoor or outdoor PCB failed (after wiring is confirmed)

How to Fix It

Power off both units at the breakers.

Turn off the indoor and outdoor breakers.
Wait 5 minutes for any board capacitors to discharge.
This is mandatory before opening either unit's electrical compartment — Daikin's F1-F2 lines can carry low voltage even when the AC is 'off' at the remote.
Check the F1-F2 wiring at the outdoor unit.

Open the outdoor unit's terminal cover.
Locate the F1 and F2 terminals.
Confirm wires are firm and not damaged.
Trace where the wires exit the unit — check for chew damage, weathering, or sections where insulation has cracked.
Replace any damaged section.
Check the F1-F2 wiring at the indoor unit.

Open the indoor unit's electrical compartment (model-specific — usually requires removing a front panel).
Confirm F1-F2 wires are seated correctly at the indoor terminal block.
Confirm polarity: F1 at indoor goes to F1 at outdoor, F2 to F2.
Reversed F1/F2 is a common installer mistake that triggers U4 on new installs.
Test F1-F2 continuity end to end.

With the system fully powered off, disconnect F1-F2 at one end.
Use a multimeter in continuity mode to test each wire from indoor to outdoor.
Both should show continuity (close to 0 ohms).
If one shows open circuit, that wire is broken — replace the run.
Verify 230V supply at both units.

Restore power.
At the outdoor unit, measure L-N supply voltage — should be 220-240V.
At the indoor unit, measure 230V at the supply side AND that the indoor PCB is showing its 'power on' indicator (varies by model).
If either side has no supply, the breaker, the disconnect, or the wiring upstream is the issue — not the AC itself.
Replace the indoor or outdoor PCB.

If F1-F2 wiring is confirmed good and both units have proper 230V supply, the PCB on one end has failed.
Outdoor PCB is the more common failure point because it's exposed to weather.
This needs a Daikin-trained technician with the model-specific replacement board.
Schedule through daikin.com.

When to Call a Professional

U4 needs an HVAC technician with electrical multimeter access to the indoor terminal block — not a homeowner job unless you're comfortable opening the indoor unit's electrical compartment safely.
If the wires between indoor and outdoor are visually damaged or chewed, replacement of the affected section is straightforward.
If wiring tests good, the next step is checking 230V supply at both ends before replacing the PCBs.

Frequently Asked Questions

I changed the indoor unit recently — could U4 be from that?

Yes — very common after an indoor unit replacement.
The most likely cause: F1 and F2 got reversed at the indoor terminal block during install.
Daikin requires F1-to-F1 and F2-to-F2 — even though the wires look identical, polarity matters on the communication bus.
Power off, reverse the two wires at one end (typically the indoor side), and power back on.
U4 clears immediately if reversed polarity was the cause.
If reversing doesn't help, check that the new indoor unit's PCB type is compatible with your outdoor — some Daikin lines mix series that don't communicate cleanly with each other.