Daikin Error Code U4: Communication Error Indoor/Outdoor
Warning: Your Daikin HVAC system has a communication error between the indoor and outdoor units. It's safe to use, but you should schedule a professional repair soon.
How to Fix Daikin Code U4
- Check for any visible damage to the communication wire connecting your indoor and outdoor units. If you find any damage, do not attempt to repair it yourself.
- If the wire appears undamaged, the issue may be with the control board or refrigerant pressure sensor. These components require professional diagnosis and repair.
How to Reset Daikin Code U4
- Turn off the unit using the remote control or wall controller.
- Turn off the circuit breaker for the outdoor and indoor units. Wait 60 seconds.
- Restore power at the breaker.
- Turn the unit on with the remote control. If the error returns, the fault needs professional diagnosis.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Daikin Error Code U4 Mean?
The Daikin U4 error code indicates a communication issue between your indoor and outdoor HVAC units. This could be due to a faulty communication wire, a problem with the control board, or an issue with the refrigerant pressure sensor. The system will continue to operate, but you may notice reduced performance or comfort.
Common Causes
- Faulty communication wire between indoor and outdoor units
- Problem with the control board in either the indoor or outdoor unit
- Issue with the refrigerant pressure sensor in the outdoor unit
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Communication wire between indoor and outdoor units is damaged or disconnected | Inspect the wire for any visible damage or disconnection | Undamaged and securely connected |
| Communication wire appears undamaged | Measure the refrigerant pressure at the service ports on the outdoor unit | Within the manufacturer's specified range for your system |
Applicable Daikin Models
- Daikin Comfort(electric)
- Daikin Fit(electric)
- Daikin Intelligent Touch(electric)
- Daikin Latitude(electric)
- Daikin Unitary System(electric)
- FIT(electric)
- Inverter Ducted Split System(electric)
- LV Series(electric)
- MXS(electric)
- Multi-Zone(electric)
- NCB(electric)
- NCM(electric)
- NCV(electric)
- NCW(electric)
- NMB(electric)
- NMP(electric)
- NPE(electric)
- NTV(electric)
- NW(gas)
- NWZ(dual_fuel)
- Quaternity(electric)
- SkyAir(electric)
- VRV(electric)
- VRV IV(electric)
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Communication Wire | — | — | — |
| Control Board | 44518C1601 | — | Find Part |
| Refrigerant Pressure Sensor | 44518C1602 | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Daikin model families as U4.
| Code | Description | Severity |
|---|---|---|
| A1 | Indoor PCB Defective | Warning |
| A3 | Drain Level System Fault | Warning |
| A5 | Freeze-Up/Overheat Protection | Warning |
| C4 | Heat Exchanger Thermistor Fault | Warning |
| E1 | Outdoor PCB Defective | Warning |
| E4 | Low Pressure Switch Activated | Warning |
| E5 | Compressor Motor Lock | Warning |
| E6 | Standard Compressor Motor | Warning |
| F3 | Discharge Pipe Temp High | Warning |
| H6 | Position Sensor Fault | Warning |