System Diagnosis
Malfunction of transmission between indoor and outdoor unit
Daikin VRV
Error Code U4
Troubleshooting guide for the Daikin VRV 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 vrv context before acting on a generic code description.
- Start with line voltage, low voltage, and board output checks in sequence so the fault is isolated cleanly.
- Inspect connectors, relay outputs, and any recently replaced electrical parts for mismatch or loose fit.
- 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
Missing voltage, inconsistent outputs, or a fault that changes when a connector is reseated.
What can fool you
Intermittent power quality and field wiring problems can mimic a failed board.
When to Escalate the Call
- You have unstable supply power, repeated fuse failures, or signs of board damage.
- The failure path involves multiple boards or field-modified wiring.
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