LG Heat Pump Error Code E420

E420 indicates a sump temperature digital sensor error, specifically an open or short circuit condition in the sensor circuit. The control board detects this fault when the sensor voltage rises above 4.95V (open circuit, corresponding to below -30°C) or drops below 0.5V (short circuit, corresponding to above 151°C). This error only triggers when the outdoor temperature is above -10°C. The heat pump stops operating to protect the compressor until the sensor issue is resolved.

You should not attempt to bypass or ignore the E420 error. The sump temperature sensor protects the compressor from operating under unsafe thermal conditions. Running the unit without a functioning sensor removes that protection and risks serious compressor damage, which is far more costly to repair than the sensor itself.

A power reset may temporarily clear the error code, but if the underlying sensor fault is still present, E420 will return once the unit attempts to operate and the outdoor temperature is above -10°C. Resetting without addressing the root cause, such as a failed sensor, damaged wiring, or loose connector, will not result in a permanent fix and could delay proper diagnosis.

Some causes of E420 are accessible to a knowledgeable DIYer. Checking whether the sensor connector has separated from the PCB and reseating it requires no special tools. Measuring resistance across the sensor terminals with a multimeter can confirm a failed sensor. However, if the PCB itself needs replacement or the fault is deep within the refrigerant system, a qualified technician should handle the repair.

If the fix is simply reseating a loose connector, there may be no parts cost at all, though a service call typically runs between $75 and $150. Replacing the sump temperature sensor itself generally costs between $20 and $80 for the part, plus labor. If the control PCB has failed, parts and labor combined can range from $300 to $700 or more depending on the model.