What It Means
Malfunction of transmission between indoor and outdoor unit.
Daikin Fan Coil
Daikin Fan Coil Error Code U4
Troubleshooting guide for the Daikin Fan Coil system fault U4.
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Possible Causes
- Short circuit in indoor-outdoor or outdoor-outdoor transmission wiring (F1 / F2), or wrong wiring.
- Outdoor unit power supply is OFF.
- System address does not match.
- Defective indoor unit PCB.
- Defective outdoor unit PCB.
Not sure which one it is — or if it's none of these? Try ACLogics today.
Disclaimer: This information is for general reference. Error codes can vary by model and production year. Always refer to your machine's specific manual and the information printed on the unit itself.
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Full Diagnosis
Malfunction of transmission between indoor and outdoor unit
All Recorded Causes
Short circuit in indoor-outdoor or outdoor-outdoor transmission wiring (F1 / F2), or wrong wiring Outdoor unit power supply is OFF System address does not match Defective indoor unit PCB Defective outdoor unit PCB
Verify Before Replacing Parts
- Confirm the exact Daikin model family and fan coil context before acting on a generic code description.
- Confirm filter condition, blower operation, and coil cleanliness before replacing any controls — start by ruling out whether short circuit in indoor-outdoor or outdoor-outdoor transmission wiring (F1 / F2), or wrong….
- Measure airflow-related readings that match the symptom, such as temperature split or static pressure, and check whether outdoor unit power supply is OFF.
- After the correction, run the system long enough to confirm the fan coil fault does not return under normal load.
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