Daikin U4 error code

Last updated 2026-08-28 · Last checked 2026-08-28 · 1629 words · 7 min read

Daikin U4 means the indoor unit has stopped receiving usable data from the outdoor unit — a transmission failure on the interconnect wiring rather than a broken compressor or fan. The unit shuts down to protect itself. The only owner-level fix is a full power reset at the switchboard; if U4 comes back, the wiring, outdoor PCB or power supply needs a licensed technician.

What does U4 mean on a Daikin air conditioner?

Daikin's Australian operation manuals list U4 as "failure of transmission between indoor unit and outdoor unit" (Cora, Alira) or "signal transmission error between indoor and outdoor unit" (Zena). It is a system-level code, not an indoor or outdoor component fault. The indoor unit expects a stream of data from the outdoor unit; when that data stops arriving or arrives garbled, the indoor PCB raises U4 and stops operation.

The service manual is more specific about the trigger: U4 is raised when "data sent from the outdoor unit cannot be received normally" or "the content of the data is abnormal". On a standard Daikin wall split the two units are joined by a three-core cable. Cores 1 and 2 carry mains power to the outdoor unit; core 3 is the signal wire. Anything that interrupts power to the outdoor unit, or corrupts the signal on core 3, produces U4.

Two things follow from that. First, U4 after a storm, brownout or a tripped breaker is often the outdoor unit simply not having powered up cleanly — a proper reset clears it. Second, a U4 that keeps returning is almost never fixed by pressing buttons; it needs someone to check the cable, the terminal connections and the two circuit boards.

What causes the U4 error?

Daikin's service manuals give five causes, in this order: wiring error on the interconnect, a broken or damaged connection wire (especially core 3), a defective outdoor unit PCB, a defective indoor unit PCB, and a disturbed power supply waveform. The ESIE16-02B manual adds power imbalance above 10 percent, a voltage drop, or a short circuit on the supply.

Some of those causes are more likely than others depending on how old the system is and what happened just before the code appeared:

Rodent damage to the interconnect cable is not in Daikin's list but is a well-known cause on Australian installations where the cable runs through a roof space or along a fence line. It presents exactly like a broken core 3.

How do I reset a Daikin U4 error?

Daikin's owner-level instruction for a blinking OPERATION lamp is to turn the power off at the circuit breaker, then restart with the remote. The service manual names the reset for U4 specifically as a "power reset via outdoor unit" — in practice, killing power to the whole system at the switchboard so both PCBs restart together. Do not just switch the remote off; that does nothing for U4.

Step by step:

1. Turn the air conditioner off with the remote.
2. At the switchboard, switch off the breaker labelled for the air conditioner (often "AC", "A/C" or "Aircon"). If the outdoor unit has its own weatherproof isolator on the wall next to it, switch that off too.
3. Wait. Daikin's documents do not specify a time; installers commonly suggest 5 to 30 minutes so the capacitors on both boards fully discharge. Five minutes is enough in most cases.
4. Switch the isolator back on, then the breaker.
5. Wait one minute, then start the unit from the remote in COOL or HEAT.
6. Listen for the outdoor unit. Within a few minutes it should hum and the fan should start. If the indoor unit runs its fan but the outdoor unit never wakes, you are looking at a power or wiring problem, not a PCB.

If the OPERATION lamp is still blinking, read the code again with the CANCEL procedure below. A different code after reset — U2 for instance — points at the power supply rather than the wiring.

How to read the code on a Daikin remote

Point the remote at the indoor unit and hold the timer CANCEL button for about five seconds until "00" blinks in the temperature display. Press CANCEL repeatedly: each press steps through the code list. A short beep or a double beep means "not this one"; a long, continuous beep means the displayed code is the stored fault. Hold CANCEL five seconds or wait one minute to exit.

That is the procedure printed in the Cora, Alira and Zena operation manuals and in Daikin's SM-TS3 service guide for ARC417, ARC423, ARC433B, ARC447, ARC452 and ARC455 remotes. The newer ARC466 and ARC480 remotes shipped with current Australian models work the same way. Very old ARC433B67/68/69/76 remotes without a CANCEL-based check use a different method: press TEMP ▲, TEMP ▼ and MODE together, then step through digits with TEMP and MODE, listening for the long beep.

On a ducted system with a BRC1E wired controller you will see "Error: Push Menu button" on the screen; pressing Menu shows the code plus the indoor and outdoor model names.

What to check before you call a technician

Everything on this list is done from outside the units with the power off at the breaker. Daikin's own manuals say not to dismantle, repair or modify the unit yourself, and in Australia the terminal block and anything under a cover is licensed work anyway. Five minutes here can save a call-out fee, or at least give the technician a head start.

1. Do the full power reset above — breaker and outdoor isolator, five minutes off.
2. Check the outdoor unit is alive. On restart, does the fan turn or the unit hum? Silence means no power or a dead outdoor board, and U4 is a symptom.
3. Look at the interconnect cable. Follow the conduit from the indoor unit's wall to the outdoor unit. Any crush damage, chew marks, exposed copper or a cable pulled out of the outdoor unit's cover is your answer. Do not open the cover.
4. Check the outdoor isolator. Some homes have a rotary isolator near the outdoor unit that can be knocked off by a mower, a gardener or kids.
5. Note what happened before. Storm, blackout, solar installation, fence repairs, roof work. Tell the technician — it shortens the diagnosis.
6. Write down the code and the model number from the sticker on the side of the indoor unit (for example FTXV50WVMA) before you ring anyone.

If U4 clears and stays clear for a week, treat it as a one-off supply glitch. If it returns within days, book the technician rather than keep resetting — a failing PCB can take other components with it.

Repair or replace?

For a system under seven years old, a U4 caused by wiring or a terminal is a cheap repair and a PCB is a moderate one. Daikin Australia's parts warranty on most residential splits is five years, so check your purchase date first — a PCB fault inside that window should cost you nothing but the call-out.

Once a unit passes about ten years, weigh a board at the top of the cost band above against the price of a new 2.5–5 kW split installed. Efficiency has moved on, R32 refrigerant units are cheaper to service than older R410A models, and a new installation restarts the warranty clock. The running cost calculator shows what a newer, higher-rated unit would save per summer in your state.

Related Daikin codes

U4 sits in Daikin's "system" group alongside U0 (refrigerant shortage), U2 (voltage drop or main-circuit overvoltage) and UA (indoor–outdoor combination fault). U2 is the one most often confused with U4 because both appear after power problems: U2 means the voltage itself was wrong, U4 means the data link failed.

Indoor-side codes that can accompany a communication problem include A1 (indoor PCB) and A5 (freeze-up or high-pressure protection, usually a filter problem). Outdoor-side codes to know are E1 (outdoor PCB), E5 (compressor overload), E7 (outdoor DC fan motor) and H0 (compressor sensor). The full list with meanings is on the All Daikin error codes page.

Frequently asked questions

Can I keep using the air conditioner with a U4 error?

No — the unit stops itself when U4 is raised, and it will not run until the fault clears or the system is reset. Running the indoor fan on its own does no harm, but repeated resets with the code returning each time are a sign to stop and book a technician.

Does U4 mean the compressor or the outdoor unit is dead?

Not necessarily. U4 only says the indoor unit cannot hear the outdoor unit. That can be a supply glitch that costs nothing, a short cable repair, or a circuit board at the top of the cost band above. The tell-tale is whether the outdoor unit powers up at all after a reset: if the fan spins and it still shows U4, the signal wire or a PCB is the suspect.

Why did U4 appear after a storm or blackout?

Because the outdoor and indoor boards restarted at different moments, or the outdoor unit never came back at all. Daikin lists "disturbed power supply waveform" and power drop as causes. A full power reset with a few minutes off usually clears it; if your breaker also tripped, have an electrician check why.

Is U4 covered by Daikin's warranty?

If the cause is a faulty PCB or an installation wiring error, generally yes within the warranty period, provided the system was installed by a licensed installer and registered. Rodent damage and damage from other trades are not warranty items. Daikin Australia's warranty terms are on the warranty card that came with the unit.

Can a multi-split show U4 on just one room?

On MXM/MKM multi-split systems U4 refers to the link between the outdoor unit and the indoor unit that displays it, so one head can show U4 while others run. A related code, U7, is used for communication problems between outdoor-unit boards on larger systems.