The Daikin U4 error code means the indoor and outdoor units are not communicating correctly. The air conditioner may stop cooling or heating, its operation light may blink, or the outdoor unit may remain inactive.
Start by checking whether there has been a power outage and whether the air conditioner’s dedicated circuit breaker is on. You may perform one controlled power restart if your model’s operating manual permits it. If U4 returns, leave the system off and arrange professional diagnosis.
Do not remove electrical covers, touch connecting terminals or test internal wiring. Stop using the system immediately if there is a burning smell, damaged cable, water leakage, unusual noise or a breaker that trips repeatedly. Daikin warns that continued operation under abnormal conditions may create an electric-shock or fire risk.
Daikin U4 Quick Answer
| Item | Answer |
|---|---|
| Error code | U4 |
| Confirmed meaning | Communication or signal-transmission failure between the indoor and outdoor units |
| Appliance | Daikin split, multi-split, SkyAir, VRV or selected heat-pump systems |
| Likely starting points | Outdoor-unit power loss, interconnecting wiring fault or unstable power |
| Other possible causes | Indoor PCB, outdoor PCB, outdoor fan motor or electrical interference |
| Safe first check | Check for a power outage and confirm the dedicated breaker has not been switched off |
| Reset allowed? | One model-approved power restart may be attempted |
| Can the system still be used? | Do not repeatedly operate or reset it while U4 persists |
| Repair difficulty | Usually requires professional electrical diagnosis |
| Call a technician when | U4 returns after one restart, the breaker trips or the outdoor unit remains inactive |
What Does the Daikin U4 Error Code Mean?
Daikin defines U4 as a failure or malfunction of transmission between the indoor and outdoor units. On a residential split system, the indoor unit sends and receives operating information through an interconnecting electrical circuit. U4 appears when expected information from the outdoor unit is missing or abnormal.
This does not identify one failed part. It tells the technician that communication has broken down somewhere in the system.
Possible explanations include:
- The outdoor unit has no power.
- The indoor-to-outdoor wiring is incorrect, damaged or disconnected.
- The supply voltage is too low or outside specification.
- Electrical interference is corrupting the communication signal.
- An indoor or outdoor control board has failed.
- An outdoor fan-motor electrical fault is affecting the control circuit.
Daikin’s general error-code chart also lists incorrect transmission wiring, an outdoor power supply that is off, an address mismatch and defective indoor or outdoor PCBs as possible U4 causes.
Is U4 a refrigerant fault?
No. U4 is not Daikin’s refrigerant-shortage code.
Daikin identifies U0 as refrigerant shortage and U4 as indoor-to-outdoor transmission failure. A separate refrigerant problem could exist, but adding refrigerant is not the standard response to a U4 code.
Avoid any contractor who recommends refrigerant charging based only on U4 without completing electrical and communication checks.
Common Causes of a Daikin U4 Fault
The exact cause depends on the model, installation and circumstances surrounding the failure.
| Possible cause | What may point toward it | Safe homeowner action |
|---|---|---|
| Outdoor unit has lost power | Outdoor unit is silent and all connected zones show errors | Check for a local outage and view the breaker position |
| Breaker or isolator switched off | Fault followed maintenance or electrical work | Confirm accessible controls have not been left off |
| Interconnecting wiring fault | Code appeared after installation, renovation or cable damage | Do not touch the cable; report the timing to the installer |
| Low or unstable voltage | Fault followed an outage, brownout or storm | Wait for the electricity supply to stabilise |
| Electrical interference | U4 appears intermittently near other heavy electrical equipment | Record when it happens and what equipment was running |
| Indoor PCB fault | Indoor unit cannot process or receive normal data | Professional diagnosis required |
| Outdoor PCB fault | Outdoor unit has power but does not communicate | Professional diagnosis required |
| Outdoor fan-motor fault | Code appears during startup or after the outdoor fan attempts to run | Turn the system off and request service |
1. No power at the outdoor unit
The indoor unit may have power while the outdoor unit does not. This can happen when:
- A dedicated outdoor breaker is off.
- An accessible isolator was left off after service.
- A fuse or protective device has operated.
- There is a power-supply problem affecting only part of the installation.
Daikin’s official U4 chart specifically lists the outdoor-unit power supply being off as a possible cause.
A breaker that has tripped is different from one that was simply switched off. Do not keep resetting a tripping breaker. It may be responding to an electrical fault.
2. Incorrect or damaged interconnecting wiring
The units communicate through wires installed between the indoor and outdoor equipment. Incorrect terminal arrangement, a disconnected conductor, damaged insulation or a broken signal wire may interrupt the data.
This is especially relevant when U4 appears:
- Directly after a new installation
- After moving or replacing a unit
- Following building or roof work
- After pest or weather damage
- After control-board or fan-motor replacement
Daikin’s residential service documentation lists misconnected wiring and a broken indoor-to-outdoor signal connection among the possible U4 causes.
These wires should be inspected only by someone qualified to work on the equipment. Depending on the design, the terminals may carry mains voltage as well as communication signals.
3. Low voltage or power-supply disturbance
A severe voltage drop may prevent one unit from operating or communicating correctly. Daikin also recognises disturbed supply waveforms and electrical noise as possible communication-fault causes.
The timing can provide a clue. Note whether U4 appeared after:
- A local power cut
- Flickering lights
- A storm or lightning
- Generator operation
- Electrical work in the building
- Startup of large motors or other heavy equipment
On selected SkyAir systems, Daikin documents intermittent U4 or U5 stops caused by electrical noise from equipment outside the air-conditioning system. Operation may restart after the interference disappears.
4. Indoor or outdoor PCB fault
A printed circuit board, commonly called a PCB or control board, manages system operation and communication. A failed board can prevent valid data from being sent or received.
A board should not be replaced simply because U4 is displayed. The technician should first establish that:
- Correct power reaches both units.
- The interconnecting conductors are correct and intact.
- There is no shorted connected component.
- The supply is stable.
- The replacement board, where applicable, has the correct configuration.
Guessing between an indoor and outdoor PCB can result in an expensive repair that does not solve the original fault.
5. Outdoor fan-motor electrical fault
On selected residential systems, Daikin includes a short circuit inside the outdoor fan-motor winding among the possible causes of U4. A defective motor can affect the outdoor control circuit and interrupt normal communication.
Do not attempt to spin the fan, disconnect its plug or open the outdoor unit. The fan may start unexpectedly, and the equipment contains hazardous electrical components.
Safe Checks You Can Perform
These checks do not require opening the indoor or outdoor unit.
1. Record the complete code
Photograph the display or write down everything shown, including:
- U4
- Any numbers following U4
- Other codes displayed before or after it
- Which indoor units are affected
- Whether the operation light is blinking
A detailed code such as U4-01 may be more useful to the technician than U4 alone.
2. Check for a general power problem
Confirm whether:
- Other appliances have lost power.
- Lights are flickering or unusually dim.
- Neighbours are experiencing an outage.
- The fault began directly after electricity was restored.
Allow an unstable supply to settle before attempting one restart.
3. View the dedicated breaker
Look at the clearly labelled air-conditioner breaker without removing the panel cover.
- If it is on, leave it on until you are ready to perform the single reset below.
- If it is visibly off and no one intentionally switched it off, check whether maintenance or electrical work occurred.
- If it is in a tripped position, do not reset it repeatedly.
Stop and call a technician or electrician when the breaker trips again.
4. Check an accessible isolator externally
Some systems have an outdoor isolation switch. Confirm only that its external handle has not been left in the off position.
Do not open the isolator, remove screws or touch its wiring.
5. Look for visible damage from a distance
Without touching the equipment, check for:
- A visibly damaged cable
- Storm or impact damage
- Signs of rodents
- Water entering electrical areas
- Scorching or melted plastic
- Vegetation or debris pressing against exposed cable
Visible electrical damage means the system should remain off.
6. Check whether one or all zones are affected
On a multi-split system, note whether:
- Every indoor unit shows U4
- Only one indoor unit is affected
- Some units work while others do not
This information helps the service provider narrow the fault to a shared outdoor unit, one branch connection or a particular indoor unit.
How to Reset a Daikin U4 Error Safely
A reset may clear a temporary communication interruption, but it cannot repair damaged wiring, a failed board or an electrical fault.
Daikin’s room-air-conditioner guidance says to turn the power off at the circuit breaker and restart the system. If the operation lamp continues blinking, the user should check the error code and consult the dealer.
Use this process once:
- Stop the air conditioner with the remote controller or normal control.
- Switch off the correctly identified air-conditioner circuit breaker.
- Confirm that the indoor display and lamps turn off.
- Follow any waiting instruction stated in the operating manual for your exact model.
- Restore the breaker.
- Start the system normally.
- Watch whether U4 or the blinking operation lamp returns.
Do not repeat the reset cycle several times. Repeatedly removing and restoring power can hide an intermittent fault without correcting it.
Do not reset the system when:
- The breaker has tripped repeatedly.
- There is a burning or electrical smell.
- A cable looks damaged.
- Water is leaking near electrical parts.
- The outdoor unit is making an abnormal sound.
- There are scorch marks or signs of overheating.
- The code appeared after known electrical damage.
Daikin instructs users to turn off the circuit breaker and call the dealer when there is a burning smell, damaged power cord, abnormal sound, repeated breaker operation or indoor-unit water leakage.
What the Timing of the Fault May Suggest
The timing does not prove the cause, but it can guide the investigation.
U4 appeared immediately after installation
Possible areas for professional checking include:
- Incorrect terminal arrangement
- Incomplete interconnecting wiring
- Outdoor power not commissioned
- Incorrect system addressing
- Incompatible equipment or setup error
Daikin uses other codes, including UA or UF on some systems, for certain combination, setup or test-run faults. Record the exact display rather than assuming every installation error is U4.
U4 appeared after a power outage or storm
The fault may involve:
- Temporary voltage instability
- A surge-damaged control board
- A protective device that has operated
- Electrical noise
- Loss of power to the outdoor unit
One controlled restart may be reasonable when there are no warning signs. A returning code needs diagnosis.
U4 appears only occasionally
Intermittent faults can result from:
- A loose or deteriorating connection
- Voltage drops
- Electrical interference
- Moisture entering a connection point
- A component beginning to fail
Record the date, outside weather, operating mode and other electrical equipment running at the time.
U4 appeared after repair work
Tell the service company what was replaced or disconnected. Possible areas include:
- Incorrectly reconnected wiring
- A connector not fully seated
- An unsuitable replacement board
- Missing model or capacity configuration
- A connected motor or component affecting the new PCB
The original installer should recheck the work before more parts are replaced.
What a Technician May Need to Test
The following checks explain the professional diagnostic process. They are not homeowner instructions.
A qualified technician may need to:
- Verify the correct supply voltage at both units.
- Confirm that the outdoor unit is receiving power.
- Compare terminal connections with the model-specific wiring diagram.
- Test the interconnecting cable for continuity, shorts and insulation problems.
- Assess the indoor-to-outdoor communication signal.
- Check for power distortion or external electrical interference.
- Inspect the outdoor fan motor and connected loads.
- Evaluate diagnostic indicators on the indoor and outdoor PCBs.
- Confirm system addressing and configuration on commercial or multi-unit systems.
- Test the indoor and outdoor control boards only after other causes are excluded.
Daikin’s service flow includes internal voltage measurements, PCB indicators, wiring checks and fan-motor assessment. These procedures involve exposed electrical components and should not be attempted by an unqualified homeowner.
Parts That May Be Involved
A U4 code may involve one or more of the following:
| Component | Possible relationship to U4 |
|---|---|
| Interconnecting cable | May be incorrectly connected, broken, shorted or damaged |
| Terminal connection | May be loose, corroded or incorrectly arranged |
| Indoor control PCB | May fail to receive or process outdoor-unit data |
| Outdoor control PCB | May fail to power up or transmit valid data |
| Outdoor fan motor | An electrical short may disrupt the outdoor control circuit |
| Power-supply component | May prevent stable voltage from reaching the control system |
| Central or branch control equipment | Relevant to selected commercial and multi-unit installations |
Do not order a PCB using the error code alone. Exact parts depend on both indoor and outdoor model numbers, production revision and region.
Daikin U4 Variations by System
| Display | Product type | General meaning | Important note |
|---|---|---|---|
| U4 | Many residential split and multi-split systems | Indoor-to-outdoor transmission failure | Causes and wiring arrangements vary by model |
| U4-01 | Selected VRV systems | Transmission error between indoor and outdoor units | Follow the model-specific U4 diagnostic flow |
| U4-03 | Selected VRV systems | Communication error involving an indoor unit and the system | Daikin directs technicians to check the indoor-unit error |
| U4-00 | Selected Daikin Altherma systems | Indoor/outdoor communication problem | Applies to hydronic heat-pump equipment, not every wall-mounted AC |
| U4-66 | Selected gateway-equipped systems | Temporary communication code during a supported firmware update | It should disappear after the update and communication restart |
| UF | Selected SkyAir or VRV installations | Setup, check-operation or transmission-wiring problem | Do not treat UF as universally identical to U4 |
Daikin VRV documentation distinguishes U4-01 from U4-03 rather than using one general code for every communication route.
Selected Altherma manuals use U4-00 for an indoor/outdoor communication problem.
Daikin UK states that U4-66 can temporarily appear during a DRGATEWAYAA firmware update because the gateway stops communicating with the user interface. Normal communication should resume after the update completes.
Can You Continue Using the Air Conditioner?
A persistent U4 code means the system cannot reliably coordinate indoor and outdoor operation. In many cases, normal cooling or heating stops automatically.
Do not repeatedly force the unit to run. Leave it off and arrange service when:
- U4 returns after one controlled restart.
- The outdoor unit remains inactive.
- The operation lamp continues blinking.
- The breaker trips.
- Several indoor units lose operation.
- The fault follows installation or electrical work.
- There is visible damage or an abnormal smell.
An intermittent code that clears by itself should still be reported if it happens repeatedly. Electrical noise, voltage problems and deteriorating connections can be difficult to find after the event.
When to Contact a Daikin Installer or HVAC Technician
Arrange professional service when any of the following applies:
- The U4 code returns after one reset.
- The outdoor unit appears to have no power.
- A breaker or safety switch trips.
- The error began immediately after installation.
- The system shows U4 with a numerical subcode.
- More than one indoor unit is affected.
- The code follows a storm, surge or building electrical fault.
- A PCB, motor or cable was recently replaced.
- There is a burning smell, abnormal sound, water leakage or visible cable damage.
Provide the technician with:
- The complete error code
- Indoor-unit model number
- Outdoor-unit model number
- Number of connected indoor units
- Which units are affected
- When the fault first appeared
- Whether a power outage or storm occurred
- Details of recent installation or repair work
- A photograph of the display
The full model number matters because Daikin’s diagnostic procedures, wiring arrangements and subcodes differ across residential, commercial and hydronic systems. Daikin UK’s technical-support guidance also advises having the full model number ready when requesting help.
Frequently Asked Questions
How do I clear the U4 error on a Daikin air conditioner?
Turn the system off normally, isolate it with the correctly identified circuit breaker and restart it once according to the model’s operating manual. If U4 or the blinking operation light returns, stop resetting the unit and contact a qualified technician.
Can a power outage cause Daikin U4?
Yes. A power outage, voltage reduction or unstable power supply may interrupt communication or prevent the outdoor unit from operating. U4 that persists after the supply has stabilised requires further diagnosis.
Does U4 mean the outdoor PCB is faulty?
Not necessarily. An outdoor PCB is one possible cause, but Daikin also lists power loss, incorrect wiring, broken connection wires, an indoor PCB fault, a fan-motor winding short and power-supply disturbance.
Can low refrigerant cause a U4 code?
Low refrigerant is not the defined meaning of U4. Daikin uses U0 for refrigerant shortage and U4 for indoor-to-outdoor transmission failure.
Why does U4 return after a reset?
The reset may temporarily restore communication, but an underlying wiring fault, voltage problem, electrical interference, motor fault or defective control board will cause the code to return.
Can I inspect the communication wire myself?
You may look for obvious external damage without touching the cable. Do not open covers, access terminal blocks or perform electrical tests. Interconnecting terminals can expose hazardous voltage.
What does U4 mean when the outdoor unit is not running?
The outdoor unit may have lost power, failed to initialise or stopped communicating because of a wiring, PCB or connected-component fault. A technician must determine which condition applies.
Is U4-66 the same as a normal U4 fault?
Not always. Daikin states that U4-66 can appear temporarily on selected systems during a network-gateway firmware update. Outside that specific update situation, record the complete code and contact the installer.
Emily Carter is an Australian home-appliance safety editor with 9 years of experience researching washers, air conditioners, refrigerators, and coffee machines. She checks troubleshooting steps for accuracy, regional model differences, and homeowner safety before publication.