System Diagnosis
Malfunction of transmission between indoor and outdoor unit
Daikin | Fan Coil
Error Code U4
Troubleshooting guide for the Daikin Fan Coil system fault U4.
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.
Potential Causes & Solutions
- 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.
- Measure airflow-related readings that match the symptom, such as temperature split or static pressure.
- After the correction, run the system long enough to confirm the code does not return under normal load.
Field Signals to Confirm
What usually confirms it
Weak airflow, abnormal temperature split, or repeated protective trips tied to blower performance.
What can fool you
Charge and control faults often look similar until airflow and coil condition are verified first.
When to Escalate the Call
- The code returns after airflow and cleanliness checks are corrected.
- You find evidence of duct or static issues that require a larger system review.
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