Daikin U4 fault codewhat it means and how to fix it
The indoor and outdoor units have stopped talking. Daikin’s own cause list starts with the mains supply and the inter-unit cable, so one breaker cycle is worth a try before anyone is called.
Take care
Which Daikin appliance is showing U4?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
A wall-mounted air conditioner with a remote control
This page is yours. Daikin’s cause list starts with the supply and the inter-unit cable, and the indoor unit is fed from the outdoor unit, so a lit indoor unit means the outdoor unit has power.
Switch off at the breaker or isolator, wait a minute, restore power once, then call an engineer with the code if U4 returns.
An Altherma heat pump with radiators or a hot water cylinder
Daikin publish the same U4 for the Altherma, but the units and the wiring are that appliance’s own and the checks belong to it.
Use the Daikin U4 page in the heat-pumps section of this site instead.
What U4 means
U4 is a failure of transmission between the indoor unit and the outdoor unit, published by Daikin under the SYSTEM heading. The two halves of a split system are constantly in communication over a cable that runs alongside the refrigerant pipes, and when that conversation stops the indoor unit reports it. It helps to know how a Daikin split is wired, because it is the opposite of what most people assume. The mains supply goes to the outdoor unit, and only the outdoor unit — Daikin’s installer guide for the RXM outdoor units carries a warning not to connect the power supply to the indoor unit. The indoor unit is then fed from the outdoor unit over the same three-core inter-unit cable that carries the communication signal.
Two things follow. An indoor unit that lights up and takes commands has not lost its outdoor unit to a switched-off isolator or a tripped breaker, because that would have taken the indoor unit down with it. And a fault in that one cable can produce U4 while everything still appears to have power. Daikin’s service manual gives the supposed causes as power supply voltage out of specification or reduced, a disturbed power supply waveform, a wiring error, a break in the connection wires between the indoor and outdoor units (they single out wire number 3, the signal core), a defective outdoor PCB, a defective indoor PCB, and a short circuit inside the fan motor winding. Notice that the first three are about the electricity coming into the house rather than either unit: a poor or noisy supply, brownouts and a neighbouring load starting up all appear on this list before any component.
Mitsubishi are explicit about a related point on their own equipment — they name the system having been powered up incorrectly as a cause of their indoor-outdoor communication fault and give repowering in the correct sequence as the remedy. That is an installer action rather than an owner one, but it explains a pattern worth recognising: a communication fault that appears immediately after electrical work in the house, when nothing was wrong with the air conditioning before. Two related codes sit nearby in Daikin’s SYSTEM group and are worth distinguishing, because they get confused with this one. UA is an indoor-outdoor unit combination fault, which is about the two units being incompatible or wrongly configured rather than unable to talk, and it is a commissioning problem. U2 is a voltage fault — a drop in voltage or main circuit overvoltage — which points at the electrical supply rather than at either unit, and may be a question for an electrician.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| A poor, low or disturbed mains supply | Three of Daikin’s eight published causes: supply voltage out of specification, a reduction in supply voltage, and a disturbed supply waveform. A brownout or a large load starting nearby can produce it, and it clears when the supply does. | Common |
| A fault in the inter-unit cable | Daikin name a wiring error and a break in the connection wires between the units, singling out wire number 3. The cable runs outside alongside the refrigerant pipes and is exposed to weather, rodents and anybody working on the wall it passes through. | Occasional |
| The system powered up in the wrong sequence | Named by Mitsubishi on their own equipment, with the remedy of repowering in the correct sequence, indoor then outdoor. Explains a comms fault that appears right after unrelated electrical work. | Occasional |
| A short in the fan motor winding | On Daikin’s list, and the reason a communication code can have a mechanical cause. A failing fan motor loads the outdoor board enough to corrupt what it sends. | Occasional |
| A circuit board fault at either end | The genuine hardware case, and the last two entries on Daikin’s list. It is where this ends up once the supply and the wiring have been eliminated, and it is engineer work. | Occasional |
What to try yourself
Retrieve the code from the remote before you do anything else, so that you have it written down whatever happens next.
Ask whether anyone has done electrical work, decorating or outside work recently, and whether the lights have dipped or a large appliance has been added. A comms fault that appears straight afterwards usually has a mundane explanation.
Look along the visible run of pipework and cable outside for obvious damage, without touching it.
Switch the system off at the breaker or isolator, wait a minute, and restore power. This is the breaker cycle Daikin’s own manual gives for a unit that has stopped with the lamp flashing.
Once only. If the breaker has tripped rather than being switched off, a device that trips again is detecting a fault, and resetting repeatedly removes the protection rather than the problem.If the code returns, call an engineer with the code. Distinguish it from UA and U2, which look similar and mean different things.
Get an F-Gas certified heat pump engineer to look at your Daikin U4
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£0–£380
Time on site
35 minutes
Nothing if it was the supply and a breaker cycle cleared it. An engineer visit to trace a wiring fault, and considerably more if a circuit board at either end needs replacing.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Daikin fault codes
- U0Get it checkedRefrigerant shortage. The circuit is sealed, so if it is short it has leaked — and this is certified work by law, not a top-up.Related: same brand
- A5Get it checkedFreeze-up protection or high pressure control. A filthy indoor filter is a legitimate published cause, which makes this the one on the list worth checking yourself.Related: same brand
- E6Get it checkedThe compressor failed to start. It sits in Daikin’s outdoor unit group alongside the overload and pressure codes, and all of them are certified work.Related: same brand
- 00Usually simple to fixNot a fault. Daikin publish 00 as NORMAL, and it is also what shows while you are stepping through the diagnostic.Related: same brand
Own a Daikin air conditioner? Save it and its codes come first next time.
Related guides
Common questions about Daikin U4
The indoor unit works, so how can the outdoor one have no power?
On a Daikin split it cannot, and that is worth knowing. The mains supply goes to the outdoor unit, and the indoor unit is fed from it through the inter-unit cable — Daikin’s installer guide warns against connecting the supply to the indoor unit at all. So if the indoor unit is lit, the outdoor unit has power, and U4 is telling you about the supply quality, the cable or a board rather than a switched-off isolator.
What is the difference between U4, UA and U2?
U4 is the two units failing to communicate. UA is a combination fault, meaning the indoor and outdoor units are mismatched or wrongly configured, which is a commissioning issue. U2 is a voltage problem — a drop in voltage or an overvoltage on the main circuit — which points outside the equipment at the supply.
Can I check the cable myself?
You can look at the visible run for obvious damage, and that is worth doing. What you cannot do is disconnect or test it. It carries the indoor unit’s mains supply as well as the signal, and it terminates inside electrical enclosures at both ends.
It cleared itself after a power cut.
That fits an interruption rather than a fault — the units lost contact, reported it, and re-established it when power was restored. Daikin list a reduced or disturbed supply among the causes. Worth keeping an eye on. A comms fault that keeps returning is a wiring or board problem and will not stay away.
I have a Daikin Altherma heat pump showing U4, not an air conditioner.
Same code, same headline: Daikin publish U4 as an indoor-to-outdoor transmission failure on the Altherma too, and Daikin’s cause list still starts with the supply and the wiring. But the units, the wiring and the power-up sequence are the Altherma’s own, and so are the checks. The Daikin U4 page in our heat-pumps section is written for it.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.