Daikin Air Conditioner Error Code U4
The indoor and outdoor units have stopped talking to each other — the most common Daikin code of all. Before suspecting boards, check the simple cause Daikin lists: the outdoor unit having no power.
Applies to: Residential split and multi-split systems
Common causes
- Short circuit or wrong wiring in the indoor–outdoor transmission wiring (F1/F2)
- Outdoor unit power supply is off
- System address does not match
- Defective indoor unit PCB
- Defective outdoor unit PCB
What to check, in order
- Switch the unit off at the breaker or isolator, wait five minutes, then switch it back on and run it again.
- Check that the outdoor unit's own isolator switch, usually on the wall beside it, is on, and that its breaker has not tripped.
- If the code returns after these checks, book a Daikin service engineer and tell them the code and the model name from the unit's label.
Clear the code and test
- Fix what the checks above found first — a filter cleaned, an obstruction removed, the breaker restored. Clearing the code without fixing the cause only brings it back.
- Restore power at the breaker and switch the unit on with the remote.
- Run the unit in cooling or heating for ten to fifteen minutes and watch whether the code returns; some codes only appear after the compressor has run for a while.
- If the unit runs with no code, the fault is cleared. If the code returns, write down the code and whether the indoor or outdoor unit shows it before calling a technician — it speeds up the diagnosis.
When to call a professional
Most cases are solved by the checks above. Book a technician if any of these apply:
- The code comes back after you have run the checks and a test cycle.
- You smell burning, see sparks or scorch marks, or the breaker trips.
- Water is leaking and you cannot stop it.
- The fix needs panels opened beyond what your manual describes, or a part tested with a meter, and you are not set up for that.
When you call, give the brand, the code U4, your model number (on a label on the side of the indoor unit) and the checks you already ran. It shortens the visit.
Parts that may be involved
Indoor unit PCBOutdoor unit PCB
Parts are model-specific. Find your model number first — usually on a label on the side of the indoor unit — and match the part to that number before ordering.
Questions people ask about U4
What does U4 mean on a Daikin air conditioner?
U4 means: Indoor–outdoor transmission (communication) error. The indoor and outdoor units have stopped talking to each other — the most common Daikin code of all. Before suspecting boards, check the simple cause Daikin lists: the outdoor unit having no power.
Can I fix Daikin U4 myself?
Start yourself: the checks on this page cover the causes you can reach without opening the machine up. If they point to a faulty part, testing or replacing it is usually a technician job.
How do I clear Daikin U4?
Fix what the checks above found first — a filter cleaned, an obstruction removed, the breaker restored. Clearing the code without fixing the cause only brings it back. Restore power at the breaker and switch the unit on with the remote. Run the unit in cooling or heating for ten to fifteen minutes and watch whether the code returns; some codes only appear after the compressor has run for a while.
Sources checked
The meaning and checks for this code were checked against the sources above in November 2026. Codes vary by model and region; if your manual lists a different meaning for your exact model, follow the manual.