LG Heat Pump Error Code E421

E421 indicates an open or short circuit fault in the SUMP TEMP FIXED 1 sensor on the outdoor unit. The system detects this error when the sensor voltage exceeds 4.95V (open circuit, reading below -30°C) or drops below 0.5V (short circuit, reading above 151°C). The fault is only registered when outdoor temperatures are above -10°C. The heat pump shuts down to protect the compressor until the sensor issue is corrected.

No, you should not continue running the unit with an active E421 fault. The sump temperature sensor helps protect the compressor from operating under unsafe conditions. Running the system without valid sensor data risks compressor damage, which is a costly repair. Address the sensor fault before resuming normal operation.

A power reset may temporarily clear the E421 error, but the code will return if the underlying sensor fault is still present. Because the system checks sensor voltage continuously during operation, any persistent open or short circuit condition will retrigger the fault. A reset is useful for confirming whether the fault is intermittent, but it is not a repair and should not be used as a substitute for proper diagnosis.

A knowledgeable DIYer can start by checking whether the sensor connector is firmly seated on the PCB and inspecting the wiring for visible damage. Measuring resistance across the sensor terminals requires a basic multimeter and can confirm whether the sensor has failed. Replacing the sensor or PCB is more involved and may require a certified HVAC technician depending on local regulations and comfort level with refrigerant systems.

If only the sensor connector needs reseating or a wire needs repair, costs may be limited to a service call fee, typically in the range of $75 to $150. Replacing the SUMP TEMP FIXED 1 sensor itself usually falls between $100 and $300 including parts and labor. If the outdoor PCB is at fault, replacement costs can range from $300 to $700 or more depending on the unit model and parts availability.