U4 Daikin Error Code | How to Troubleshoot U4 Error

The U4 error code on a Daikin system typically means a communication error between the indoor and outdoor units. This is common in split and VRV/VRF systems.

❗ What “U4” Means on Daikin Units

• U4 = Transmission Error

  Communication between the indoor and outdoor units (or among system components) has failed or been interrupted.

🔍 Common Causes of U4 Error

🛠️ How to Troubleshoot U4 Error

✅ 1. Check Power to All Units

• Make sure both indoor and outdoor units are receiving proper voltage.

• Turn off the breaker for 5 minutes, then turn it back on to reset the system.

✅ 2. Inspect Communication Cable

• Look for loose, cut, reversed, or corroded wires.

• On most Daikin systems, this is a 2-core communication wire (often terminal S1/S2).

• Ensure proper polarity and that terminals are tight.

✅ 3. Ensure Proper Startup Sequence

• Power should be applied to outdoor unit first, then indoor unit after a few seconds.

• Improper sequencing can lead to a U4 fault.

✅ 4. Check for Interference

• Nearby electronics, poor grounding, or overlapping signal lines may interfere with communication.

✅ 5. Call a Professional

• If wiring and power are correct, the issue could be a failed PCB (printed circuit board).

• Daikin-trained HVAC techs can run full diagnostics and may need a specialized service tool.

🧰 For VRV / VRF Systems

On larger commercial Daikin systems (VRV/VRF), a U4 code may appear on:

• Outdoor unit PCB

• Service checker display

• Central controller like I-Touch or Intelligent Touch Manager

In that case, the same general troubleshooting applies, but may require service software or dip switch resets.

📝 Summary