Daikin Error Code U4: Transmission Error Between Indoor and Outdoor Unit
Communication failure between indoor unit PCB and outdoor unit PCB on transmission terminals F1/F2 or 1-2-3.
Equipment TypeInverter AC & VRV / Heat Pump CommunicationBrand: Daikin
Fault SeverityCriticalSystem Lockout Active
Estimated Repair Cost$120 – $550Parts & Certified Labor
🚨 Symptoms of Daikin Error U4
- AC or heat pump stops completely and green/red LED blinks
- Operation lamp on controller flashes U4
- Neither cooling nor heating functions
🔍 Root Causes for Daikin Code U4
- Loose or disconnected F1/F2 or 1/2/3 communication interconnecting cable
- Indoor or outdoor PCB failure / blown fuse
- Electromagnetic noise interference on signal wiring
- Mismatched indoor/outdoor unit combination or incorrect dip switch settings
🛠️ How to Reset & Troubleshoot Daikin Error U4
- Turn OFF main electrical isolator for 5 minutes to hard-reset PCBs
- Check DC voltage between communication wires (terminals F1-F2 should oscillate between 15V-30V DC)
- Inspect outdoor unit terminal block for loose screw terminals or corrosion
- Replace outdoor unit main inverter PCB if communication circuit is blown
Frequently Asked Questions — Daikin Code U4
What does Daikin error code U4 mean?
Daikin code U4 indicates: Communication failure between indoor unit PCB and outdoor unit PCB on transmission terminals F1/F2 or 1-2-3.
How do I reset error code U4 on Daikin?
Turn off the electrical circuit breaker or isolator to the unit for at least 5 minutes to discharge the capacitors and reset the microprocessor. If the code reappears immediately upon restarting, professional inspection is required.
What causes error code U4 on Daikin systems?
Common triggers include: Loose or disconnected F1/F2 or 1/2/3 communication interconnecting cable, Indoor or outdoor PCB failure / blown fuse, Electromagnetic noise interference on signal wiring.