Daikin U4 Error Code

What does Daikin U4 mean?

The U4 code means the indoor and outdoor units have lost communication over the interconnect bus. Daikin runs a low-voltage serial link on terminals F1/F2 between the indoor and outdoor PCBs, and when the outdoor PCB stops hearing the indoor (or vice versa) beyond the timeout window, both units report U4. Wiring faults at the F1/F2 terminals, incorrect polarity on two-wire systems, and PCB communication circuits are the three failure paths.

Symptoms

• U4 displays on the indoor controller at power-up or during operation
• Neither indoor nor outdoor unit responds to run commands
• Fault clears briefly after a 30-second power-off then returns
• On multi-zone systems, only one indoor unit reports U4 while others run
• LED indicators on the outdoor PCB show no transmission activity

Common causes

• Broken, shorted, or miswired interconnect cable between indoor and outdoor
• Loose terminal connection at F1/F2 on either unit
• Mixed or crossed power and communication wiring
• Defective indoor or outdoor PCB communication circuit
• Incorrect address or DIP-switch setting after recent service

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Diagnostic steps

1. Power-cycle both units together

   Isolate indoor and outdoor disconnects for 30 seconds, then restore. This clears one-off communication lock-ups.

2. Inspect F1/F2 terminals

   Open the terminal covers on both units and confirm the interconnect wires are seated tight on the correct terminals with no stray strands.

3. Check cable continuity

   Isolate power, disconnect one end of F1 and F2, and continuity-test each conductor end to end. Look for shorts between wires or to ground.

4. Verify no power on the communication pair

   With power on, voltage between F1/F2 should match the service-manual value. Full 230 V or zero indicates miswired power.

5. Confirm address and DIP settings

   On multi-unit systems, review the addressing per the commissioning sheet. A duplicate address will cause U4.

6. Replace the suspect PCB

   Swap the outdoor PCB first — if U4 persists, swap the indoor PCB. Reconfigure address and DIPs after replacement.

When to call a professional

Call a licensed technician if U4 persists after verifying wiring and power-cycling. On VRV and multi-zone systems, commissioning addresses, branch-selector addressing, and termination settings must match the design; wrong settings produce U4 on apparently sound hardware. Running 230 V on the communication pair destroys PCBs on both ends fast, so wiring verification before powering back on matters. A Daikin-trained tech can read the outdoor PCB diagnostic LEDs to localise the fault.