The Daikin controller on the wall is flashing U4. The system won’t respond to any commands — no heating, no cooling, fan won’t start. The facility manager tries powering it off and on from the controller, but U4 comes straight back every time. Staff are asking questions, the building is uncomfortable, and nobody on site knows what U4 means.
U4 is one of the most common error codes on Daikin VRV and VRF systems. It indicates a communication fault between the indoor and outdoor units. The system can’t operate if the units can’t talk to each other. The good news: U4 is usually a wiring or connection issue rather than a catastrophic component failure. The bad news: on a multi-unit VRV system, tracking down a communication fault requires methodical diagnosis.
What U4 Actually Means
Daikin VRV systems use a two-wire serial communication bus connecting every indoor unit back to the outdoor unit. This bus carries operational data in both directions — temperature readings, mode commands, compressor demands, error reports, and system status. When an indoor unit loses communication with the outdoor unit, the system raises a U4 error and shuts down the affected circuits as a protective measure. No communication means no coordinated operation, and running without coordination risks equipment damage.
The scope of the U4 error tells you a lot about the fault location. U4 on a single indoor unit suggests a problem with that unit’s wiring or PCB. U4 across multiple indoor units simultaneously points to a fault on the main communication trunk, at the outdoor unit’s comms board, or on the shared section of the bus.
Loose Wiring Terminations
This is the number one cause of U4 errors across all Daikin VRV installations. The communication bus connects via screw terminals at every indoor unit and at the outdoor unit. Over years of continuous operation, vibration — particularly from the outdoor unit’s compressor — gradually loosens these connections. A termination that was properly torqued during installation can be barely making contact after three or four years of service.
The fix is simple in principle: check and re-tighten every termination on the communication bus. Start at the unit displaying the error and work outward. The outdoor unit terminations deserve particular attention because they experience the most vibration. Use a torque screwdriver set to the manufacturer’s specification — overtightening damages the terminal block just as surely as undertightening causes intermittent connections.
Damaged Communication Cable
Recent building works are the biggest risk factor here. Ceiling tile replacement, electrical rewiring, cable tray modifications, partition installation, fire alarm upgrades — any work in the ceiling void or risers can damage the VRV communication cable. We’ve seen cables nicked by electricians routing new containment, crushed during ceiling grid reinstallation, and chafed through where they run over sharp bracket edges. Sometimes the damage is obvious. Sometimes it’s a partial nick in the insulation that causes intermittent faults only when the cable moves or the temperature changes.
If U4 appeared shortly after building work, inspect the cable route between the affected indoor unit and the outdoor unit. Look for visible damage, new cable ties that are compressing the comms cable, or areas where the cable has been disturbed. Damaged communication cable should be replaced in full sections — splicing VRV comms cable is unreliable and not recommended by Daikin.
Indoor Unit Address Settings
Every indoor unit on a Daikin VRV system has a unique address on the communication bus. On older systems, these addresses are set using DIP switches on the indoor unit’s PCB. On newer systems, addresses are assigned through commissioning software via the system controller. If an address gets corrupted — after a power surge, a PCB replacement, or an accidental DIP switch change during maintenance — the outdoor unit can’t identify that indoor unit on the bus. Communication fails, U4 appears.
If a unit’s address has been corrupted or duplicated, it needs resetting to its correct value. On DIP switch systems, this means physically accessing the indoor unit PCB and verifying the switch positions against the commissioning record. On software-addressed systems, the commissioning tool reconnects and reassigns the address. Either way, the system needs reinitialisation after an address correction to rebuild the communication map.
PCB Failure on Indoor or Outdoor Unit
The communication interface circuitry on either an indoor or outdoor unit PCB can fail independently of other board functions. A power surge, moisture ingress into the control enclosure, or simple component aging can damage the comms transceiver while leaving other circuits intact. A failed comms chip on an indoor unit’s PCB means that specific unit goes silent on the bus. A failed comms section on the outdoor unit’s main PCB can knock out communication with every indoor unit on the system.
Diagnosis involves systematically isolating units from the communication bus. Disconnect indoor units one at a time and check whether the remaining units can communicate. If disconnecting a specific indoor unit restores communication for everything else, that unit’s PCB is the culprit. If no combination of indoor unit disconnections restores communication, the outdoor unit’s comms board is likely at fault. PCB replacement resolves the issue — provided the correct board is available.
Power Supply Issues Causing Communication Dropout
The communication bus requires stable power to function reliably. If the supply voltage to an indoor unit drops below the operating threshold — due to a loose power connection, a degraded transformer, or an overloaded electrical circuit — the unit’s PCB may not have enough power to sustain the communication signal. The unit appears powered on — LEDs illuminated, display active — but it can’t maintain the bus communication protocol. The result is an intermittent U4 that comes and goes, often correlating with other electrical loads switching on and off in the building.
Check the incoming power supply voltage at every unit displaying U4. Compare the measured voltage to Daikin’s specified operating range for that model. Low voltage or voltage that sags under load points to a power supply fault rather than a communication fault — fixing the electrics fixes the U4.
Earth Fault on the Communication Line
An earth fault occurs when the communication cable’s insulation breaks down and a conductor contacts earthed metalwork — a cable tray, a pipe, a structural beam. This corrupts the communication signal across the entire bus, potentially causing U4 errors on multiple or all indoor units simultaneously. Earth faults are often intermittent: the cable touches metalwork when the building warms and expands, loses contact when it cools. A system that works fine on Monday morning but throws U4 every afternoon is a classic earth fault pattern.
An insulation resistance test on the communication cable — measured section by section between each pair of units — will locate an earth fault. The affected cable section needs replacing, and the route should be inspected to prevent the same damage recurring.
ADK and Daikin U4 Repairs
U4 is the Daikin VRV error code we see most often, and it’s typically resolved on the first visit. Our engineers carry replacement PCBs for common Daikin indoor and outdoor units, along with the manufacturer’s diagnostic interface to interrogate the bus, read detailed fault histories, and pinpoint exactly where communication has broken down. Most U4 repairs are completed within a couple of hours.
If your Daikin VRV system is flashing U4, call ADK on 0207 801 0808. We’ll find the fault, fix it, and get your system communicating again.