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Daikin Error Code U4 - Transmission Error Between Indoor and Outdoor Units

Medium Severity

The microprocessors in the indoor unit and the outdoor condensing unit have lost serial data communication across terminal wires 1, 2, and 3.

Appliance: Daikin Air Conditioners & Heat Pumps

Estimated Time: 20 minutes

Tools Required: Screwdriver, multimeter

⚠ Common Symptoms & Observations

Air conditioner stops cooling/heating; operation light on indoor unit blinks green; outdoor compressor and fan do not start.

🔍 Root Causes for Code U4

Loose or corroded communication wire connection on Terminal 3
electrical noise interference
blown fuse on the outdoor PCB
faulty outdoor inverter printed circuit board.

🔧 Step-by-Step DIY Troubleshooting & Fix Guide

Turn off the main electrical breaker supplying power to both indoor and outdoor units for 10 minutes.

Inspect the 3-core interconnecting cable between units. Check that wire #3 (data line) is tightened firmly in its screw terminal with no frayed strands.

Restore breaker power and turn the unit on with the remote controller.

🔄 Master Reset Procedure for Daikin

Using the Daikin remote control: Press and hold the 'CANCEL' button for 5 seconds until '00' blinks. Press 'CANCEL' repeatedly until a long, continuous beep sounds to read and clear active fault codes.

Technician Tip: If the error code reappears immediately after a hard reset, the control board has detected an active electrical open circuit or failed component that requires physical inspection.