What causes Daikin Mini Split U4?

The outdoor unit has no power, for example a tripped breaker or a switched-off disconnect at the outdoor unit. Daikin's code chart lists this directly. A wiring error, or a loose, broken or poorly insulated wire in the indoor/outdoor connection cable (on ductless models Daikin points at the signal wire, terminal No. 2 or No. 3 depending on the model). On VRV systems, a short circuit, disconnection or wrong wiring on the F1/F2 transmission line, or a system address that doesn't match. A disturbed power supply waveform (electrical noise) interfering with the signal. A defective outdoor unit control board. On some ductless models Daikin also lists a short circuit inside the outdoor fan motor winding, which can take the board down with it. A defective indoor unit control board.

How do I fix Daikin Mini Split U4?

Check whether the outdoor unit has power. Look for a tripped breaker in the panel and make sure the outdoor disconnect switch is on, since an unpowered outdoor unit is a listed U4 cause. Do a full power reset: switch off every breaker feeding the indoor and outdoor units, wait a few minutes, then switch them back on and try cooling or heating again. Give the system several minutes after the reset. On VRV systems Daikin notes the system may not operate for up to 12 minutes after a reset. Look along the outside line set for obvious damage to the cable, such as chewed wires, crushed covers or recent digging, trimming or construction nearby. Think about what changed. U4 right after installation, a board replacement or wiring work points toward the connection wiring, so call the installer first. Don't open the terminal covers or test the connection wires yourself. On Daikin ductless systems the indoor/outdoor cable carries line voltage along with the signal.

When should I call a pro for Daikin Mini Split U4?

U4 returns after a full power reset with the outdoor unit powered. A technician checks the indoor/outdoor connection wires for wiring errors, breaks and insulation, which is Daikin's first check. The wiring is good. On ductless models Daikin's flow checks the blink of LED A on the outdoor board, the voltage between the connection terminals and the power supply waveform before deciding which board to replace. The outdoor fan doesn't turn smoothly by hand. Daikin's flow for some ductless models says to replace both the fan motor and the outdoor unit board. Every indoor unit on a VRV system shows U4 and the F1/F2 wiring checks out. Daikin's remedy is replacing the indoor unit board indicated by the error code; if boards or wiring were changed recently, a technician resets power and holds the reset button on the master outdoor board for 5 seconds. The code appeared after a storm or new electrical equipment on the same circuit, since a disturbed power supply is a listed cause and may need an electrician. The system is new or under warranty. Contact the installing contractor or a Daikin dealer first.

Code: U4🔧 Daikin Mini Split

Daikin Error Code U4: Indoor and Outdoor Units Can't Communicate (Meaning, Causes, Fixes)

Q: What does Daikin Mini Split U4 mean?

U4 on a Daikin system is a transmission (communication) error between the indoor and outdoor units. The indoor unit isn't getting normal data from the outdoor unit, so the system stops. Daikin's service manuals list wrong, loose, broken or shorted connection wiring, the outdoor unit's power being off, a disturbed power supply, and a faulty indoor or outdoor control board. A full power reset and a check that the outdoor unit actually has power are the owner steps; wiring and board checks need a technician. Daikin's ductless (wall-mounted FTX/FTXS-type) service manuals call U4 "Signal Transmission Error (between Indoor Unit and Outdoor Unit)": the indoor unit checks the data it receives from the outdoor unit, and U4 appears when that data can't be received normally or its content is abnormal. On VRV and SkyAir commercial systems the same code is "Transmission Error between Indoor Units and Outdoor Units" (shown as U4 or U4-01 in monitor mode), set when transmission over the F1/F2 line isn't carried out normally for a certain amount of time. Daikin's quick malfunction code chart adds that U4 can simply mean the outdoor unit's power supply is off. Older Daikin manuals show the same fault as U4 or UF. It is a communication fault, not a refrigerant or compressor code, though a failing part on the outdoor board side can be the reason the signal stops.

Reviewed 2026-10-24 · Based on 5 manufacturer & service sources

What the code means

Daikin's ductless (wall-mounted FTX/FTXS-type) service manuals call U4 "Signal Transmission Error (between Indoor Unit and Outdoor Unit)": the indoor unit checks the data it receives from the outdoor unit, and U4 appears when that data can't be received normally or its content is abnormal. On VRV and SkyAir commercial systems the same code is "Transmission Error between Indoor Units and Outdoor Units" (shown as U4 or U4-01 in monitor mode), set when transmission over the F1/F2 line isn't carried out normally for a certain amount of time. Daikin's quick malfunction code chart adds that U4 can simply mean the outdoor unit's power supply is off. Older Daikin manuals show the same fault as U4 or UF. It is a communication fault, not a refrigerant or compressor code, though a failing part on the outdoor board side can be the reason the signal stops.

Likely causes

The outdoor unit has no power, for example a tripped breaker or a switched-off disconnect at the outdoor unit. Daikin's code chart lists this directly.
A wiring error, or a loose, broken or poorly insulated wire in the indoor/outdoor connection cable (on ductless models Daikin points at the signal wire, terminal No. 2 or No. 3 depending on the model).
On VRV systems, a short circuit, disconnection or wrong wiring on the F1/F2 transmission line, or a system address that doesn't match.
A disturbed power supply waveform (electrical noise) interfering with the signal.
A defective outdoor unit control board. On some ductless models Daikin also lists a short circuit inside the outdoor fan motor winding, which can take the board down with it.
A defective indoor unit control board.

Safe DIY checks

⚠ Before you start

Turn off the breakers for both the indoor and outdoor units before any inspection. Daikin's manuals warn to turn off the power switch before connecting or disconnecting connectors. The connection cable between Daikin ductless units carries line voltage, and the outdoor board sits beside high-voltage inverter parts that can stay charged after power is off, so leave terminal blocks and boards to a licensed HVAC technician.

Check whether the outdoor unit has power. Look for a tripped breaker in the panel and make sure the outdoor disconnect switch is on, since an unpowered outdoor unit is a listed U4 cause.
Do a full power reset: switch off every breaker feeding the indoor and outdoor units, wait a few minutes, then switch them back on and try cooling or heating again.
Give the system several minutes after the reset. On VRV systems Daikin notes the system may not operate for up to 12 minutes after a reset.
Look along the outside line set for obvious damage to the cable, such as chewed wires, crushed covers or recent digging, trimming or construction nearby.
Think about what changed. U4 right after installation, a board replacement or wiring work points toward the connection wiring, so call the installer first.
Don't open the terminal covers or test the connection wires yourself. On Daikin ductless systems the indoor/outdoor cable carries line voltage along with the signal.

🛠 When to call a pro

U4 returns after a full power reset with the outdoor unit powered. A technician checks the indoor/outdoor connection wires for wiring errors, breaks and insulation, which is Daikin's first check.
The wiring is good. On ductless models Daikin's flow checks the blink of LED A on the outdoor board, the voltage between the connection terminals and the power supply waveform before deciding which board to replace.
The outdoor fan doesn't turn smoothly by hand. Daikin's flow for some ductless models says to replace both the fan motor and the outdoor unit board.
Every indoor unit on a VRV system shows U4 and the F1/F2 wiring checks out. Daikin's remedy is replacing the indoor unit board indicated by the error code; if boards or wiring were changed recently, a technician resets power and holds the reset button on the master outdoor board for 5 seconds.
The code appeared after a storm or new electrical equipment on the same circuit, since a disturbed power supply is a listed cause and may need an electrician.
The system is new or under warranty. Contact the installing contractor or a Daikin dealer first.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Indoor/outdoor connecting cable

Broken or miswired connection wires are Daikin's first U4 suspect; a technician should match the wire type to the installation manual.

Search eBay →

⚙

Outdoor unit control board

Replaced when Daikin's diagnosis points at the outdoor PCB; match the exact outdoor model number before ordering.

Search eBay →

⚙

Multimeter

Used by the technician to check voltage between the connection terminals.

Search eBay →

Sources

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.

Daikin Error Code U4: Indoor and Outdoor Units Can't Communicate (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Indoor/outdoor connecting cable

Outdoor unit control board

Multimeter

Related codes

Sources