What it means
Daikin's service manuals call this a signal transmission error between the indoor and outdoor unit. Daikin's European user guides list it under system errors with the same wording.
What to check, in order
- Read and note the code
Use the timer cancel check on the remote and write the code down, along with the full model name.
- Switch off at the air conditioner's own breaker
Daikin's troubleshooting page says to switch off the air conditioner's circuit breaker and wait 20 minutes. Use the breaker for the air conditioner, not the main switch for the whole house.
- Switch back on and restart
Turn the breaker back on and start the unit with the remote. If the operation lamp flashes again, Daikin says the unit is likely to be faulty.
When to call an engineer
Call your installer or an air conditioning engineer (HVAC technician in the US) if it comes back. Daikin's service manual lists a supply voltage that is out of specification or dropping, a wiring error, a broken connecting wire between the units, a faulty outdoor or indoor control board, a short circuit in the fan motor winding and a disturbed power supply waveform.
Quick answers
What does the Daikin U4 error code mean?
The indoor unit is not receiving normal signals from the outdoor unit, so the system has stopped. Daikin's service manuals call this a signal transmission error between the indoor and outdoor unit. Daikin's European user guides list it under system errors with the same wording.
Can I fix Daikin U4 myself?
Easy (about 25 minutes). Start with: read and note the code; switch off at the air conditioner's own breaker; switch back on and restart. Call your installer or an air conditioning engineer (HVAC technician in the US) if it comes back. Daikin's service manual lists a supply voltage that is out of specification or dropping, a wiring error, a broken connecting wire between the units, a faulty outdoor or indoor control board, a short circuit in the fan motor winding and a disturbed power supply waveform.
Is Daikin U4 worth repairing?
Worth fixing. A breaker reset can clear a one-off glitch. If the code returns, the wiring, power supply or a control board needs an engineer.
Checked against
- Daikin Service Manual SiUS042113E, FTXM-V / RXM-V inverter pair wall mounted type (manualslib copy of the Daikin PDF)
- Daikin Service Manual SiUS04-924_A, RXS / FTXS H-series inverter pair wall mounted type (PDF)
- Daikin: Simple Self-Diagnosis by Malfunction Code, SM-TS3, list of error codes (PDF)
- Daikin Global: Troubleshooting, split and multi-split air conditioners
- Daikin CTXA / FTXA User reference guide 4PEN518786-10V, English (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.