Daikin U4 Error Code: What It Means and How to Fix It

A Daikin showing U4 has not lost refrigerant, and its compressor has not failed. The indoor unit has stopped receiving usable data from the outdoor unit, so it shuts down rather than run without knowing what the other half of the system is doing.

That much is common knowledge. What almost no guide gets right is the detail that decides how long the repair takes: which wire Daikin actually uses for that signal, and what the digits after the dash mean when the code arrives as U4-01 instead of plain U4. Both are published by Daikin, and both are below.

What Daikin’s manuals actually say U4 means

Daikin defines U4 twice, once for each side of its product range, and the two definitions are not interchangeable.

Daikin’s global malfunction code list marks U4 as applicable to every product family it publishes: RA, SkyAir, VRV, packaged, heat reclaim ventilator, chiller and fan coil. That breadth is exactly why generic advice about it goes wrong. The same three characters cover a wall-mounted head in a bedroom and a 40-ton VRV system, and the diagnostic path is different.

The causes Daikin lists differ too. For a residential system: a wiring error, a break in the connection wires between indoor and outdoor units, a defective outdoor unit PCB, a defective indoor unit PCB, or a disturbed power supply waveform. For VRV and SkyAir: a short circuit or wrong wiring in the F1/F2 transmission wiring, the outdoor unit power supply being off, a system address that does not match, or either PCB being defective.

That last residential cause, the power supply waveform, has no equivalent in any homeowner guide, and it is the reason a U4 can follow a generator swap, a failing UPS, or a nearby piece of equipment putting noise on the supply.

Read the code properly first, using the Service Check Function

On a Daikin wall-mounted unit the code is not on a display. You have to ask the remote for it, and the procedure is specific enough that people give up halfway and misread the result.

Using an ARC452-series remote:

1. Hold the timer cancel button down for 5 seconds. 00 appears on the temperature display.
2. Press timer cancel repeatedly. The code indication steps through Daikin’s list in a fixed order, and U4 is the second entry, right after 00.
3. Keep going until you hear a long beep. That is the code the unit is actually reporting.
4. A short beep, or two consecutive beeps, means the code you are sitting on is not the one stored. Keep pressing.
5. Hold timer cancel for 5 seconds to leave. Left untouched for 60 seconds, the remote exits on its own.

Daikin notes that not every error code is displayed by this method. If your code never produces a long beep, the unit needs the second check method from the service manual, which is a technician procedure.

U4-01, U4-03, U4-15: the digits after the dash decide the job

On VRV and SkyAir, U4 arrives with a two-digit suffix, and the suffix is not decoration. It changes which unit you work on.

• U4-01 is the indoor-outdoor transmission error. This is the classic U4, and Daikin’s own note against it in the error code list is simply “refer to the U4 flow chart”.
• U4-03 is a transmission error between the indoor unit and the system. Daikin routes this one away from the U4 flow entirely: read the indoor unit’s own error code and troubleshoot that. Treating a U4-03 as a wiring job is how an afternoon disappears.
• U4-15 means the outdoor unit could not start test operation because one of the indoor units detected an error. Nothing is wrong with the transmission line at all; an indoor unit is holding up commissioning.

There is a fourth pointer worth knowing. U3-05, a premature end of test operation during an initial transmission error, explicitly tells the technician to go and check the indoor unit’s U4 or U9 error. A U3 on the outdoor unit and a U4 on an indoor unit during commissioning are the same event seen from two ends.

The wire that fails is No. 3, not S1, S2 or S3

This is where most of the internet is wrong about Daikin, and it is worth being blunt.

Daikin’s residential service manuals label the interconnecting terminals No. 1, No. 2 and No. 3, and name the communication conductor as wire No. 3. Daikin’s VRV and SkyAir manuals label the transmission pair F1 and F2. Neither range uses S1, S2 and S3.

S1 / S2 / S3 is Mitsubishi Electric’s terminal labelling. If a page tells you to inspect the S1/S2/S3 wiring on a Daikin U4, it has been written from a Mitsubishi E6 article with the brand swapped, and a technician following it will be looking for terminals that are not on the unit.

The practical version:

• Residential Daikin. The fault is in the connection wires between indoor and outdoor, with wire No. 3 carrying the signal. Daikin’s own test measures the terminal board voltage between No. 1 and No. 3, and between No. 2 and No. 3.
• VRV and SkyAir. The fault is on the F1/F2 pair, and it can be either the indoor-outdoor run or an outdoor-outdoor run between modules, which is a length of cable most people never think to check.

What a homeowner can safely check

Two things, and they resolve a real share of U4 events.

1. Power-cycle once. Turn the system off at the remote, switch its breaker off for about a minute, then restore power. A transient transmission glitch clears here and does not come back.
2. Confirm the outdoor unit actually has power. “Outdoor unit power supply is OFF” is on Daikin’s own cause list. A tripped breaker or an outdoor disconnect switch left open presents to the indoor unit as silence, which is indistinguishable from a broken wire. If the outdoor unit is dead rather than quiet, our guide to an outdoor unit not running is the faster route.

If U4 returns after that, stop resetting it. Repeated power cycling of a system with a damaged conductor or a failing board achieves nothing and hides how often the fault is occurring.

Daikin’s own diagnostic flow, step by step

This is the residential flow chart from Daikin’s service manual, in order. It is here so you know what a competent technician will do, and can tell whether the one in your house is doing it.

1. Inspect the indoor-outdoor connection wires for a wiring error. If one is found, correct it and the flow ends here.
2. Check LED A on the outdoor unit PCB. Two preconditions are easy to miss and produce a false reading if skipped: cancel the standby electricity-saving function first by starting fan operation from the remote, and wait at least 7 seconds after turning the power on. If LED A is not blinking, the outdoor unit PCB is diagnosed next.
3. Measure the connection wire voltage on the indoor terminal board, between No. 1 and No. 3, and between No. 2 and No. 3.
4. Check the insulation of those connection wires. If they are not properly insulated, they are replaced.
5. Check the power supply waveform. Daikin’s Check No. 11 measures between No. 1 and No. 2 on the terminal board and asks two questions: is the waveform a sine wave, and is there disturbance near the zero cross? A disturbed supply is located and corrected rather than treated as a unit fault.
6. If everything above is clean, the indoor unit PCB is replaced.

The order matters. A board swap performed before step 5, on a system with a disturbed supply, will produce a second U4 on a brand-new board.

Same code, wrong fix: U4 next to U3, UF and UH

Daikin’s U-series is dense, and three of its members get mistaken for U4 routinely, especially on a system that has just been installed or worked on.

The one that costs the most time is UF. It is widely repeated online that UF is “the same as U4, seen during installation”. Daikin’s malfunction code list says otherwise: UF is system is not set yet, and one of its listed causes is that the stop valve has not been opened. A system that has never been commissioned, and a system whose service valves are shut, are two different jobs, and neither is a wiring hunt.

After a board swap or a rewire: the BS3 address reset

On VRV, Daikin’s flow chart asks a question no consumer guide asks: has the indoor or outdoor unit PCB been replaced, or has the transmission wiring been modified?

If yes, the system’s stored addresses no longer match the hardware, and the fix is not a wiring hunt, it is a re-registration. Daikin’s instruction is to press and hold BS3 on the master outdoor unit main PCB for 5 seconds or more, after which the system will not operate for up to 12 minutes while it re-learns its own layout. That silence is expected behaviour, not a second fault.

If no board was replaced and no wiring changed, the flow instead resets the power supply once and reads the detail of the error in monitor mode, which is where the suffix comes from.

The flow chart’s first question on a multi-head system is also worth repeating, because it splits the job in half before any tool comes out: do all the indoor remote controllers in the same refrigerant system display U4, or only one? All of them points at the shared transmission run, the outdoor power supply or the outdoor board. Only one points at that indoor unit’s board, or its own leg of the wiring.

When to call a professional

Call once a power cycle has failed and the outdoor unit is confirmed to have power. From that point Daikin’s own procedure requires opening the electrical box, reading LED A, and taking voltage measurements on an energised terminal board, and on VRV, entering monitor mode to retrieve the suffix.

Tell whoever comes three things: whether every indoor head shows the code or only one, whether the suffix is 01, 03 or 15 if your system displays one, and whether any board was replaced or any wiring altered since it last worked. Those three answers place the fault in Daikin’s flow chart before anyone opens a panel.

The full Daikin code set, and the other codes this one is mistaken for, are on our Daikin mini split error codes page.