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Daikin Altherma fault U4-00

Indoor/outdoor unit communication problem

Heat pump

What it means

Communication failure between outdoor and indoor unit; unit will stop operating.

Likely causes

  1. Faulty or disturbance of the power supply (power supply MUST be within range of nominal operating voltage ±4%)
  2. Faulty wiring between the outdoor unit, indoor unit and domestic hot water tank
  3. Faulty main PCB
  4. Faulty hydro PCB
  5. Communication error between bizone PCB and current loop PCB
  6. LD2 flashing: TCP/IP communication problem with router
  7. LD3 flashing: communication problem between LAN adapter and indoor unit

Where to start

  1. Check if the power supply is compliant with the regulations
  2. Perform a check of the power supply, connections, wiring between the outdoor unit, indoor unit and (separate) domestic hot water tank (if applicable)
  3. Perform a check of the main PCB

The full answer, free

  • 9 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 58), right under the answer
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Safety: Isolate the outdoor unit and any backup heater, and give the inverter capacitors time to discharge before you open the electrics. Anything on the refrigerant circuit needs F-gas certification.

From the Daikin manual for the Altherma. Check the model on the data plate before you order parts.

Other Daikin Altherma fault codes