Daikin Heat Reclaim Ventilator Error Code U3

Full Diagnosis

Indicates the mandatory commissioning check operation (test run) was never completed after installation or a system reset, or that a transmission error interrupted it. Daikin HRVs require this initial check run so the control PCB can auto-detect indoor/outdoor unit combinations, damper positions, and connected accessories before normal ventilation operation is permitted; until it completes successfully the unit will not enter its normal automatic mode. Unlike U4, which flags an active transmission failure between indoor and outdoor units during normal running, U3 is specifically about the commissioning step itself not having been executed or having failed partway through.

All Recorded Causes

Check/test operation was never run after installation, a PCB replacement, or a factory reset Check operation was started but interrupted before completion (power loss, wiring disturbed mid-run) Transmission wiring between indoor and outdoor units not yet properly connected at the time check operation was attempted Stop valves or dampers not in the correct position for the check run to proceed

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 check/test operation was never run after installation, a PCB replacement, or a factory res….
3. Inspect connectors, relay outputs, and any recently replaced electrical parts for mismatch or loose fit, and verify check operation was started but interrupted before completion (power loss, wiring disturbe….
4. After the correction, run the system long enough to confirm the heat reclaim ventilator fault does not return under normal load.