Daikin furnace shows U4 and will not run; communication thermostat may be blank or show communication error.

Safety: Turn off power to the furnace and thermostat circuit before working on low-voltage wiring.

Steps to check:

• Inspect thermostat cable: Remove the thermostat from its base and the furnace blower door. Trace the low-voltage cable between the Daikin control board and thermostat. Look for staples through the cable, cuts, or rodent damage.
• Verify terminal connections: At the furnace control board, confirm that the communication terminals (often labeled A/B or D+/D− on Daikin systems) are wired consistently to the same terminals at the thermostat. Tighten any loose screws.
• Check polarity: Ensure R and C (24V power) are not reversed or shorted to other terminals. Miswiring can cause U4 communication faults.
• Bypass wall thermostat (test only): If using a non-communicating thermostat interface, temporarily jumper R to W at the furnace board to see if the furnace will start a heat call. Remove the jumper after testing. If it runs, the thermostat or thermostat wiring may be faulty.
• Power cycle: Restore power to the furnace and thermostat, wait 5–10 minutes, and see if the U4 clears.

If wiring and polarity are correct and U4 persists, a technician should test the Daikin control board and communicating thermostat/interface and replace the defective component.