Skip to main contentStart diagnosing U4 on your Daikin heat pump
What this code means
Signal transmission error between indoor unit and outdoor unit – data sent from outdoor unit cannot be received normally or content is abnormal.
First step from the service manual
Check the power supply voltage for ±10% fluctuation from rated value.
Next steps: diagnose the root cause
1
Verify the error code is active
Power cycle the unit and see if the code returns. If it does, the fault is persistent and requires diagnosis.
2
Get the full diagnostic procedure
Get step-by-step guidance, test values, and wiring diagrams from the OEM service manual.
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What’s available for Daikin heat pump models
Access Daikin heat pump service manuals, wiring schematics, exploded parts views, and error code references on MyPros+.