Daikin Mini-Split U4 Error Code: Indoor-Outdoor Communication Fault
Quick Answer
Daikin U4 is a communication fault on the 2-wire S1-S2 serial bus connecting indoor and outdoor units. The most common cause is a loose terminal screw at either the indoor or outdoor control board - Daikin uses push-in connectors on newer FTXM models that vibrate loose over time. On installations with refrigerant line runs exceeding 50 feet, voltage drop on the communication wire itself can cause intermittent U4.
Daikin U4 is a communication fault on the 2-wire S1-S2 serial bus connecting indoor and outdoor units. The most common cause is a loose terminal screw at either the indoor or outdoor control board - Daikin uses push-in connectors on newer FTXM models that vibrate loose over time. On installations with refrigerant line runs exceeding 50 feet, voltage drop on the communication wire itself can cause intermittent U4. Check S1-S2 terminal tightness at both ends first. If wiring is sound, measure DC voltage between S1-S2 during operation - it should pulse between 0V and 30V. A flat line at 0V means the outdoor board is not transmitting.
What Does the U4 Code Mean?
U4 means the indoor and outdoor units cannot communicate over the S1-S2 wiring. Check terminal connections at both units first.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Symptoms You May Notice
- U4 error code on indoor unit display
- Unit stops heating or cooling
- Indoor unit fan may still run but no compressor activity
- Outdoor unit shows no LED activity
- Error appears intermittently in windy conditions (wire movement)
Can you reset a Daikin minisplit to clear the U4 code?
Turn the circuit breaker off for at least 30 seconds, then turn it back on. After power is restored, press Cancel on the remote control for 5 seconds to clear the fault memory. If using a Daikin One+ thermostat: Menu > System > Reset.
Tools Required for Diagnosis
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
Turn off the breaker
Turn off the breaker. Open the indoor unit cover and locate the S1-S2 terminal block on the right side of the PCB. Check that both wires are firmly seated and the screws are tight. Do the same at the outdoor unit terminal block.
With power restored
Inspect the full length of the communication wire from indoor to
If wiring is confirmed good and outdoor board is not transmitting
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
| Part Name | OEM Number | Estimated Price |
|---|---|---|
| Daikin Outdoor Unit Control BoardModel-specific · $150–$350 | Model-specific | $150 – $350 |
| 2-conductor communication wire18/2 shielded · $0–$1 | 18/2 shielded | $0 – $1 |
Repair vs. Replace Calculator
Frequently Asked Questions
Can I fix Daikin U4 myself?
Why does U4 come and go on my Daikin?
How is Daikin U4 different from Mitsubishi communication errors?
Related Daikin Minisplit Error Codes
Models Known to Experience U4 Errors
This repair applies to most Daikin minisplits with this error code. Common model numbers include:
FTXS09LVJU, FTXS12LVJU, FTXS15LVJU, FTXM09QVJU, FTXM12QVJU, FTXM15QVJU, FTXM18QVJU, FTXM24QVJU
Last verified for technical accuracy on March 15, 2026