U4
Daikin • Air Conditioner
ElectricalHazard
Professional Only
Daikin AC Error Code U4: Transmission Failure Between Indoor & Outdoor Units
Verification
95% Official
Manufacturer Guide
High Voltage Electrical Hazard Warning
This error code points to inverter modules, capacitor banks, or motor stator coils that carry lethal electrical potential. Disconnect main power breaker immediately. Do NOT touch internal terminal blocks while energized.
What Does Error U4 Mean?
Communication Error (Loss of data transmission between the indoor unit PCB and the outdoor condenser unit PCB).
Diagnostic Tree & Resolution Protocol
Official manufacturer root cause analysis and step-by-step diagnostic workflow.
#1 Most Common
Wiring mismatch or loose connection on terminal block 1, 2, or 3.
Wiring mismatch or loose connection on terminal block 1, 2, or 3.
#2 Most Common
Blown fuse on outdoor inverter control PCB.
Blown fuse on outdoor inverter control PCB.
#3 Most Common
Transient power surge / lightning strike damaging inverter communication IC.
Transient power surge / lightning strike damaging inverter communication IC.
#4 Most Common
High voltage noise from nearby machinery.
High voltage noise from nearby machinery.