Daikin Heat Reclaim Ventilator

Daikin Heat Reclaim Ventilator Error Code U4

Troubleshooting guide for the Daikin Heat Reclaim Ventilator system fault U4.

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What It Means

Malfunction of transmission between indoor and outdoor unit.

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

  1. Confirm the exact Daikin model family and heat reclaim ventilator context before acting on a generic code description.
  2. Start with line voltage, low voltage, and board output checks in sequence so the fault is isolated cleanly — the data plate flags short circuit in indoor-outdoor or outdoor-outdoor transmission wiring (F1 / F2), or wrong….
  3. Inspect connectors, relay outputs, and any recently replaced electrical parts for mismatch or loose fit, and verify outdoor unit power supply is OFF.
  4. After the correction, run the system long enough to confirm the heat reclaim ventilator fault does not return under normal load.

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