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| Error Code | Fault Meaning & Symptoms | Suspected PCB Component | Diagnostic Action | Lab Repair |
|---|---|---|---|---|
| U4 | Blue Star U4 Error Indoor / Outdoor serial communication failure. Outdoor unit does not start. | Communication Optocouplers PC817 & Loop Resistors | Oscilloscope test communication pulse waveform; replace faulty optocoupler. | WhatsApp Repair Board |
| L5 | Blue Star L5 Error Inverter compressor instantaneous overcurrent trip. | IGBT IPM Module & Current Sense Shunt Resistors | Check resistance between UVW terminals; desolder blown IPM and test SMPS drive lines. | WhatsApp Repair Board |
| E6 | Blue Star E6 Error Outdoor fan motor locked or DC fan feedback missing. | DC Fan Motor Driver IC & 15V Gate Supply Line | Measure 15V and 310V rails; replace damaged fan driver microcontroller. | WhatsApp Repair Board |
| C4 | Blue Star C4 Error Indoor heat exchanger liquid pipe thermistor sensor open or short. | Temperature Sensor Bias Network & ADC Input Port | Measure thermistor resistance (typically 10k-20k ohm at 25C) and filter capacitor. | WhatsApp Repair Board |
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