HeatingCodesDaikin mini-split error U4: signal transmission error between indoor and outdoor unit
What U4 means
The indoor and outdoor units of a ductless system talk to each other over the connecting cable. Daikin's service manuals describe how U4 is detected: the signal transmission data received from the outdoor unit is checked to see whether it is normal, and U4 is set when the data cannot be received normally or the content of the data is abnormal. The AXVJU manual calls the same code "Communication abnormality."
Causes listed in each Daikin manual
| Daikin manual | Models | Causes listed for U4 |
|---|---|---|
| DAMA-SM-23-020 | FTXC series | Power supply abnormal; wrong wiring of connecting wires or wire breaking; defective indoor or outdoor PCB |
| DAMA-SM-23-006 | FTX-B / FTK-B (BXVJU) | Power supply voltage out of specification; reduction of power supply voltage; wiring mis-connection; breaking of the connection wire between indoor and outdoor units (wire no. 3 / SIG); defective outdoor PCB; short circuit inside the fan motor winding; defective indoor PCB; disturbed power supply waveform |
| AXVJU service manual | FTXB / FTXN / FTKB / FTKN AXVJU | Faulty outdoor PCB; faulty indoor PCB; signal transmission error due to a wiring error, a disturbed power supply waveform, or a broken wire in the connection wires |
| SiUS091601ED | FTX-N/U, FVXS, FDMQ with RXL | Same eight causes as DAMA-SM-23-006, with "wire No. 3" named as the connection wire |
Two of those causes are about the quality of the electricity rather than a broken part: a reduction of supply voltage and a disturbed power supply waveform. That is why U4 can appear after a storm, a brownout or a utility outage, and why a technician's first step is to measure the supply voltage.
Daikin's technician checks
- Check the power supply voltage. The AXVJU manual's U4 chart looks for voltage fluctuation within plus or minus 10% of the rated value.
- Check the indoor-outdoor connection wires for a wiring error.
- Measure the voltage on the indoor terminal board between terminals 1 (L) and SIG, and between 2 (N) and SIG (AXVJU manual), then check the LED on the outdoor unit PCB.
- Depending on the results, the charts lead to replacing the connection wires, the outdoor PCB or the indoor PCB.
All of the manuals warn technicians to turn off the power switch before connecting or disconnecting connectors, because parts can be damaged. These checks involve line voltage at the terminal boards and are not homeowner work.
What a homeowner can safely do
- Check that the breaker and the outdoor disconnect for the system are on. If the outdoor unit has no power, it cannot send signals to the indoor unit.
- Think about the last 24 hours: a power outage, a flicker, or a storm. Tell your technician, because supply-voltage problems are on Daikin's list.
- Turn the system off with the remote, wait a few minutes, and restart once.
- From outside, look along the visible part of the cable and line set for obvious damage, for example from snow removal, a fallen branch or animals. Do not touch damaged wiring.
- If U4 returns, record the model numbers of both units and call a licensed HVAC technician.
Safe homeowner checks for Daikin U4
- Check the breaker and disconnectMake sure the circuit breaker and the outdoor disconnect switch for the mini-split are on.
- Restart onceTurn the system off with the remote, wait a few minutes, then turn it back on.
- Look for visible damageFrom outside, look for damage to the visible cable and line set without touching it.
- Call a technicianIf U4 returns, call a licensed HVAC technician with both model numbers.
Frequently asked questions
Can a power outage cause Daikin U4?
Daikin lists reduction of power supply voltage and a disturbed power supply waveform among the causes, so a U4 after a storm or outage is consistent with the manual. If the code keeps coming back once power is normal, the wiring or a circuit board is more likely.
Is U4 the same on every Daikin mini-split?
In the four service manuals we read (FTXC, FTX-B, AXVJU and FTX-N/U), U4 always means a signal transmission or communication problem between the indoor and outdoor unit. Check the manual for your exact model for other series.
What is the difference between U4 and UA?
U4 is a communication error between units that were working together. UA is listed as an installation error (or "unspecified voltage") between the indoor and outdoor unit, which points at a mismatched or mis-wired combination.
Sources
- Service Manual DAMA-SM-23-020: Inverter Split Wall Mounted Heat Pump, FTXC Series (section 4.1 and 4.20) (Daikin) Covers: Daikin FTXC-series inverter wall-mounted heat pumps (60 Hz).
- Service Manual DAMA-SM-23-006: Inverter Split Wall Mounted FTK-B & FTX-B Series (section 4.1 Error Codes and Description, 4.8, 4.20) (Daikin) Covers: Daikin FTX09/12/18/24BXVJU heat pump indoor units with RX09/12/18/24BXVJU outdoor units, and cooling-only FTK-BXVJU / RK-BXVJU pairs (208/230 V, 60 Hz).
- Service Manual: Inverter Wall Mounted Single Split, FTKB/FTKN/FTXB/FTXN-AXVJU (section 3.3 Error code description for Inverter) (Daikin) Covers: Daikin FTXB09-24AXVJU / RXB, FTXN09-24AXVJU / RXN heat pumps and FTKB / FTKN cooling-only pairs (the 19 Series AXVJU wall units).
- Service Manual SiUS091601ED (03/2022): FTX-N/U wall mounted, FVXS-V floor standing and FDMQ-R duct connected with RXL outdoor units (Part 6, 4. Error Codes and Description) (Daikin) Covers: Daikin FTX09/12/15NMVJU(A), FTX18/24UVJU, FVXS09/12/15NVJU and FDMQ12/18/24RVJU indoor units with RXL09/12/15QMVJU and RXL18/24UMVJU outdoor units.
Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.