LG Heat Pump Error Code E401

E401 indicates that the outdoor ambient temperature sensor has been detected in an open circuit or short circuit state. The outdoor PCB monitors this thermistor's resistance to track outdoor temperature and control system operation. When the resistance reading falls outside the valid range, the system triggers E401. The fault can originate from a loose connector, a failed sensor, or damaged wiring.

Operating the heat pump with an active E401 error is not recommended. Without a functioning outdoor temperature sensor, the control board cannot accurately manage system behavior based on ambient conditions, which can lead to improper operation or additional component stress. It is best to address the fault before resuming normal use.

If the fix is simply reseating a loose connector, there may be no parts cost at all, though a service call fee typically ranges from $75 to $150. Replacing the outdoor temperature sensor itself is generally an inexpensive repair, with parts usually ranging from $15 to $60 and total repair costs including labor between $100 and $250. If the outdoor PCB is found to be faulty, costs increase significantly, often ranging from $300 to $600 or more depending on the model and labor rates in your area.

A homeowner comfortable with basic electrical checks can start by inspecting and reseating the sensor connector on the outdoor unit's control board, which requires no special tools. If the connector appears secure and the error persists, diagnosing and replacing the sensor requires using a multimeter to measure resistance, which is within the capability of a knowledgeable DIYer. However, if the fault traces back to the PCB itself, a qualified technician should handle that repair.

A power reset may temporarily clear the error code from the display, but if the underlying sensor fault is still present, E401 will return once the system re-evaluates the sensor circuit. A reset alone does not fix a disconnected connector, failed thermistor, or damaged wire. The root cause must be identified and corrected for the error to clear permanently.