Diagnostic Overview for Daikin Error Code U4
Reported Symptoms: Outdoor unit does not start; green LED on outdoor motherboard is dead or blinking 5 times; indoor fan runs with no cooling.
Technical Cause: Transmission failure between indoor and outdoor unit microcontrollers. High voltage communication line interrupted.
Step-by-Step Lab Repair Procedure:
Check 24V communication loop, replace optocouplers (PC817 / TLP281), inspect communication resistors (20k ohm / 2W), replace outdoor MCU if line was struck by surge.
Send your board to our dedicated testing lab via all-India courier pickup for high-voltage bench diagnostics and 90-day replacement warranty.