Home › Air Conditioners › Daikin › U4
Daikin Air Conditioner U4 Error Code
Indoor and outdoor units can't communicate. Here is what it means and how to clear it, easiest fix first.
The indoor unit has stopped receiving data from the outdoor unit, so the system shuts down. Typical causes are no power reaching the outdoor unit, interconnecting wiring that is loose, shorted or wired incorrectly, an address setup error on systems with several units, or a faulty circuit board in either unit.
On multi-zone systems, note whether every indoor head shows it or only one; that helps the technician. Daikin uses this code across its residential split, SkyAir and VRV ranges, but the exact meaning and causes can vary by series, model year and region, so confirm it in your unit's manual. How to read it: most Daikin ductless wall units only blink the operation lamp. On remotes such as the ARC433 series, press and hold the timer Cancel button for roughly 5 seconds until 00 blinks on the screen, then press Cancel again one step at a time; when the indoor unit answers with a long continuous beep, the code showing is the stored fault. SkyAir and VRV wireless remotes use a different sequence with the Inspection/Test, up/down and MODE buttons, and wired wall controllers usually show the code on their screen.
How to fix Daikin U4
Reset the power at the breaker
Switch the system off with the remote, then turn off its circuit breaker in your home's main panel. Wait about 5 minutes, switch the breaker back on and restart the unit. If the code appeared after a storm or power cut, restarting both units together sometimes clears it.
Check that the outdoor unit has power
Look for a second breaker labeled for the outdoor unit or heat pump and make sure it is on. If a breaker trips again as soon as you reset it, leave it off; that points to an electrical fault.
When to call a technician
If U4 returns after a full power reset, the wiring between the units or a control board needs testing by an HVAC technician. Don't open the outdoor unit's electrical cover or the indoor unit's terminal block yourself.
Parts that commonly cause U4
- indoor/outdoor connecting cable
- indoor control board
- outdoor control board
Order parts by your exact model number so they fit.
Other Daikin air conditioner codes
- U0Low refrigerant or restricted refrigerant flow
- U2Power supply voltage problem
- UAIndoor and outdoor units don't match
- A1Indoor unit control board fault
- A3Condensate water level too high
- A5Freeze-up or overheat protection
- A6Indoor fan motor fault
- C4Indoor coil temperature sensor fault
- C9Indoor room air temperature sensor fault
- E1Outdoor unit control board fault
- E5Compressor overload or lock-up protection
- E6Compressor won't start
- E7Outdoor fan motor fault
- E8Compressor current too high
- F3Compressor discharge temperature too high
- F6High pressure in cooling mode
- H6Compressor position sensing error
- L4Inverter heat sink overheating
- L5Inverter output overcurrent
Frequently asked questions
What does U4 mean on a Daikin air conditioner?
The indoor unit has stopped receiving data from the outdoor unit, so the system shuts down. Typical causes are no power reaching the outdoor unit, interconnecting wiring that is loose, shorted or wired incorrectly, an address setup error on systems with several units, or a faulty circuit board in either unit.
Can I fix Daikin U4 myself?
Only the basic checks. You can safely try the steps above, but U4 usually points to a part or fault that needs a technician.
What if U4 keeps coming back?
A code that returns after a reset and the checks above usually means a part is wearing out. Note when U4 appears (which cycle, how often) and have your model number ready; it helps a technician bring the right part on the first visit.
Last reviewed 2026-10-19. Codes and steps can vary by model; check your owner's manual.