Samsung Air Conditioner Error Code E421

The E421 error code on a Samsung air conditioner indicates an OPEN or SHORT circuit fault detected in the SUMP TEMP FIXED 1 sensor. This sensor monitors the temperature of the sump (oil sump or refrigerant sump) in the outdoor unit. The control board triggers this error only when the outdoor ambient temperature is above -10°C, which eliminates false positives that could occur in extreme cold. The fault is defined by voltage readings at the sensor signal circuit: a reading above 4.95V corresponds to an open circuit condition representing a detected temperature below -30°C, while a reading below 0.5V corresponds to a short circuit condition representing a detected temperature above 151°C.

The sump temperature sensor is a thermistor whose resistance changes predictably with temperature. When the circuit is open, perhaps due to a broken wire or failed sensor element, the control board reads an abnormally high voltage because no current path exists to pull the signal line down. When the circuit is shorted, the signal line is pulled to near ground, producing a near-zero voltage reading. Both conditions fall outside the valid operating window and trigger E421.

Common causes include a disconnected or corroded sensor connector, physical damage to the sensor wiring harness, a failed thermistor element inside the sensor body, or moisture intrusion into the connector causing a short. The first diagnostic step is to inspect the sensor connector for proper seating and measure resistance across both sensor terminals. A healthy NTC thermistor will show measurable resistance at room temperature; an open circuit or near-zero resistance reading confirms sensor or wiring failure.