Samsung Air Conditioner Error Code E421

E421 indicates an OPEN or SHORT error on the SUMP TEMP FIXED 1 sensor in the outdoor unit. The system detected a sensor signal voltage either above 4.95V (open circuit, below -30°C reading) or below 0.5V (short circuit, above 151°C reading). This fault is only triggered when outdoor temperature is above -10°C. The unit requires diagnosis and repair of the sensor circuit before it will operate normally.

It is not recommended to continue running the unit with E421 active. The sump temperature sensor protects compressor components from operating outside safe temperature limits. Running the system without accurate sump temperature monitoring could lead to compressor damage or other component failures.

A power reset may temporarily clear the fault display, but E421 will return as soon as the control board detects the out-of-range sensor voltage again. Since the error is triggered by a hardware condition in the sensor circuit, the underlying problem must be repaired to permanently clear the code. Do not rely on repeated resets as a workaround.

A knowledgeable DIYer can check the sensor connector for proper seating and inspect the wiring harness for visible damage. Measuring resistance across the sensor terminals with a multimeter will confirm whether the thermistor has failed. However, if the sensor or control board needs replacement, familiarity with the outdoor unit wiring and safe handling of electrical components is required, and in many areas refrigerant-side work requires a certified technician.

Repair costs depend on the root cause. Reconnecting a loose connector involves little to no parts cost, with any labor charge being the main expense. Replacing the SUMP TEMP FIXED 1 sensor typically involves parts costs in the range of $20 to $80 and one to two hours of labor. If the outdoor control board needs replacement, total repair costs can range from $200 to $600 or more depending on the model and service rates in your area.