Daikin U4/U0 Communication & Inverter Triage
A U4 error means the 15V-28V DC pulse between Terminal 2 (Neutral) and Terminal 3 (Comms) has flatlined. A U0 error is a logic deduction: the outdoor PCB sees that the EEV (Electronic Expansion Valve) is wide open, yet the compressor discharge thermistor is skyrocketing, indicating there is no cool suction gas returning to cool the compressor windings.
Daikin Remote Diagnostics Protocol
To extract the fault code from a Daikin remote (ARC452 series and similar), hold the CANCEL button for 5 seconds until “00” blinks on the screen. Press CANCEL repeatedly. A short beep indicates the first letter matches. A long, continuous beep indicates the exact 2-digit fault code.
| Fault Code | System Designation | Primary Testing Location |
|---|---|---|
| U4 | Indoor-to-Outdoor Transmission Error | Terminals 2 & 3 (DC Volts) |
| U0 | Refrigerant Shortage / EEV Fault | Discharge Thermistor / Service Valves |
| L5 | Inverter Compressor Overcurrent | IPM Board / Compressor Windings |
Diagnostic Gateway Challenge
You are diagnosing a Daikin split system with a U0 error. You connect your gauges and see the standing pressure is normal for R-32, but the compressor refuses to start. You check the discharge thermistor resistance and it reads completely open (infinite ohms). What is the actual fault?