Daikin AC Error Code U4 — Outdoor Communication Fault

What Does This Error Mean?

U4 on a Daikin AC means communication between the indoor unit and the outdoor unit has been interrupted. The two units exchange data continuously through a dedicated signal wire — when this signal is lost, U4 is triggered and the system shuts down. The most common cause is a power issue at the outdoor unit, a loose connection, or a PCB fault.

Affected Models

Daikin FTKF Series
Daikin FTKG Series
Daikin ATKF Series
Daikin Inverter Split AC (all models)
Daikin 1 Ton / 1.5 Ton / 2 Ton Inverter

Common Causes

Outdoor unit not receiving power — tripped breaker or disconnected isolator
Communication wire (S-wire or signal wire) broken, shorted, or loose at terminal block
Outdoor inverter PCB fault preventing communication
Indoor PCB communication circuit fault
Voltage fluctuation corrupting the communication signal
Incorrect wiring during installation — terminals swapped or signal wire not connected

How to Fix It

Check that the outdoor unit has power.

Look for a dedicated AC circuit breaker in your electrical panel and an isolator switch near the outdoor unit. Reset any tripped breaker.
Turn off both units at the main AC circuit breaker, wait 60 seconds, and restore power.

A full simultaneous reset of both PCBs can clear U4 caused by a power event or temporary communication dropout.
Listen for the outdoor unit starting up.

A completely silent outdoor unit after restart strongly suggests a power supply problem on the outdoor side rather than a wiring fault.
If U4 persists, call a Daikin authorised technician.

The technician will test the signal wire and measure voltages at both PCBs to determine where the communication break has occurred.

When to Call a Professional

If a power cycle does not resolve U4, an authorised Daikin technician is required. Communication circuit diagnosis requires specialist tools and access to the outdoor inverter board.

Frequently Asked Questions

Why did U4 appear after I had the outdoor unit serviced?

If U4 appeared immediately after a service visit, the most likely cause is that the technician accidentally disturbed the communication wire terminal connections while working on the outdoor unit. Contact the service company — this is their responsibility to correct at no charge.

Does U4 mean both PCBs have failed?

Not necessarily — U4 only means communication has been lost. In the majority of cases, either the wiring or only one PCB is at fault. A technician will test the signal path methodically to identify the single point of failure.