Samsung Air Conditioner Error Code E309

Error code E309 on a Samsung air conditioner indicates a fault with the discharge temperature sensor on fixed capacity compressor 2. This sensor monitors the temperature of refrigerant gas leaving the compressor, which is critical for protecting the compressor from overheating and ensuring the refrigeration cycle operates within safe parameters. The error is triggered specifically when the outdoor ambient temperature is above -10 degrees Celsius, and the sensor signal falls outside the acceptable voltage window. An OPEN condition is detected when the signal voltage exceeds 4.95V, corresponding to a sensed temperature below -30 degrees Celsius. A SHORT condition is detected when the voltage drops below 0.5V, corresponding to a sensed temperature above 151 degrees Celsius.

The discharge sensor is a negative temperature coefficient (NTC) thermistor whose resistance changes predictably with temperature. When the PCB reads a voltage outside the valid range, it cannot determine a reliable discharge temperature, so it raises E309 to prevent compressor damage from undetected overheating or other refrigerant circuit faults. The outdoor unit PCB continuously monitors this sensor signal during operation whenever ambient conditions are warmer than -10 degrees Celsius, meaning the error will not trigger during extreme cold weather.

Common causes include a disconnected or corroded sensor connector at the PCB, a damaged sensor harness with a broken or shorted wire, or a failed thermistor element within the sensor itself. Physical damage to the sensor from vibration or contact with hot compressor surfaces can also cause out-of-range readings. Diagnosing the fault starts with inspecting the sensor connector for separation or corrosion, then measuring the sensor resistance and comparing it to the expected resistance-temperature curve for the specific sensor fitted.