LG Heat Pump Error Code E123

E123 on an LG heat pump indicates an open or short circuit fault in the MCU temperature sensor. This sensor monitors the temperature of the main control board during operation. The system triggers this code when it cannot read a valid signal from the sensor, either because the circuit is broken or the sensor has shorted internally. The unit will not operate normally until the fault is resolved.

The E123 error means the system cannot verify the temperature of its main control board, which removes a key thermal protection layer. Continuing to run the unit in this condition risks undetected overheating of the MCU board, which could cause further component damage. It is best to stop using the heat pump and have the fault diagnosed before resuming operation.

A power reset may temporarily clear the E123 code, but if the underlying sensor fault still exists, the error will return once the system reinitializes and attempts to read the sensor. Resetting does not repair a failed sensor, damaged wiring, or corroded connector. A physical diagnosis and repair of the sensor circuit is necessary to permanently resolve the fault.

A knowledgeable DIYer can start by checking whether the sensor connector is fully seated and visually inspecting the wiring harness for obvious damage. Using a multimeter to measure resistance across the sensor terminals is also within reach for those comfortable with basic electrical testing. However, replacing the MCU temperature sensor or the control board typically requires disassembly of the unit and should be handled by a qualified technician if you are not experienced with appliance repair.

Repair costs vary depending on the root cause. Reseating a connector or cleaning corroded pins may only cost a service call fee, typically in the range of $75 to $150. Replacing the MCU temperature sensor itself is generally inexpensive in parts but adds labor costs. If the MCU control board needs replacement, total repair costs could range from $300 to $700 or more depending on the model and labor rates in your area.