ErrorCodeDoctor
Daikin · Air Conditioner

Daikin Air Conditioner U4 error code

Difficulty: Medium · Cost: $0–$350 · Duration: 5–10m

The U4 error means your Daikin air conditioner detected a communication failure between the indoor and outdoor units. This is most commonly caused by a wiring fault in the indoor-outdoor transmission wiring (F1/F2), incorrect wiring, or the outdoor unit's power being off. Check that the outdoor unit is powered on and that the transmission wiring is correctly connected. Contact a qualified technician if the wiring is correct.

Common causes

  • Short circuit or wrong wiring in the indoor-outdoor transmission wire (F1/F2)
  • Outdoor unit power supply is off
  • System address settings do not match
  • Indoor unit PCB has a fault
  • Outdoor unit PCB has a fault

How to fix it

  1. 1

    Check that the outdoor unit circuit breaker is on and the outdoor unit has power

  2. 2

    Check that the F1/F2 transmission wiring between indoor and outdoor units is correctly and securely connected

  3. 3

    If wiring is correct and both units have power contact a qualified technician to inspect the PCBs and system address settings

Related Daikin Air Conditioner error codes

1

A0

An external protection device connected to the indoor unit has been activated

hard
2

A1

A fault was detected in the indoor unit PCB (control board)

hard
3

A3

The drain level control system has detected an abnormality — drain may be clogged

medium
4

A5

Freeze-up protection (cooling) or high pressure control (heating) has activated — check the air filter

easy

U4 error — frequently asked questions

What does the U4 error mean on a Daikin air conditioner?

A communication fault was detected between the indoor and outdoor units

What causes the U4 error?

The U4 error is usually caused by: Short circuit or wrong wiring in the indoor-outdoor transmission wire (F1/F2); Outdoor unit power supply is off; System address settings do not match; Indoor unit PCB has a fault; Outdoor unit PCB has a fault.

Can I fix the U4 error myself?

It's possible for a moderately handy person, but budget 5–10m and expect some disassembly.

How much does it cost to fix the U4 error?

Fixing the U4 error on a Daikin air conditioner typically costs $0–$350, depending on parts and whether you do it yourself or hire a technician.

Is it safe to keep using my air conditioner with the U4 error?

In most cases yes, but it's best to address the U4 error soon — see the fix steps below to resolve it.