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U4

U4 - Malfunction of transmission between indoor and outdoor unit (Indoor/Outdoor Unit)

Daikin · Simple Self Diagnosis by Malfunction Code | Daikin

What does U4 mean?

There is a communication failure between the indoor and outdoor units.

Common Causes

  • Damaged communication wiring
  • Loose connection
  • Interference from other devices
  • Faulty control board on either unit

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Turn off power to both units.

  2. 2

    Inspect communication wiring for damage and ensure secure connections.

  3. 3

    Restore power and check if communication is re-established.

  4. 4

    If the error persists, contact a service technician.

Browse all codes in this manual (60)
Verified technical data. Last updated: April 2026

Related Faults

6-06-0 - External protection device activated (Heat reclaim ventilator)6-16-1 - Malfunction of PCB (Heat reclaim ventilator)6-26-2 - Ozone density abnormal (Heat reclaim ventilator)
Source: Simple Self Diagnosis by Malfunction Code | Daikin
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