LG Heat Pump Error Code E423

E423 indicates a fault with the SUMP TEMP FIXED 3 temperature sensor in the outdoor unit. The system has detected an OPEN or SHORT condition in the sensor circuit, meaning the signal voltage has gone above 4.95V or below 0.5V. These voltage extremes correspond to temperature readings below -30°C or above 151°C, which are outside the valid operating range. The fault is only triggered when outdoor ambient temperature is above -10°C.

Operating the heat pump while E423 is active is not recommended. The sump temperature sensor provides data the control system uses to protect the compressor and manage refrigerant conditions in the outdoor unit. Running the system without a functioning sensor could allow conditions that damage the compressor or other components. It is best to address the fault before resuming normal operation.

Repair costs vary depending on the root cause. A simple connector cleaning or reseating may cost little beyond a service call fee, typically in the range of $75 to $150. Replacing the SUMP TEMP FIXED 3 sensor itself usually falls in the $100 to $300 range including parts and labor. If the outdoor control PCB requires replacement, costs can rise to $400 to $900 or more depending on the model and labor rates in your area.

Some basic checks, such as inspecting the sensor connector for corrosion, reseating loose connectors, and visually inspecting the wiring harness, can be done by a knowledgeable DIYer. However, testing sensor resistance, tracing a wiring fault, or diagnosing a PCB issue requires a multimeter and familiarity with refrigeration system components. If the sensor or board needs replacement, a qualified technician is recommended to ensure correct parts and safe reassembly.

A power reset may temporarily clear the error display, but the code will return if the underlying sensor fault is still present. Because the system checks the sensor voltage continuously during operation, any persistent open or short condition will retrigger E423 once the unit restarts and ambient conditions meet the detection threshold above -10°C. A reset is useful for confirming whether a fault is intermittent, but it does not repair a failed sensor or damaged wiring.