LG Heat Pump Error Code E121

E121 indicates an open or short circuit fault in the MCU temperature sensor, which monitors the thermal condition of the main control board. The system detected that the sensor's electrical signal is outside the acceptable range, either showing no continuity (open) or a direct short. This triggers the fault code to alert you that the sensor circuit needs inspection and repair.

Continuing to operate the unit with an active E121 fault is not advisable. The MCU temperature sensor provides thermal protection data for the control board, and without a reliable reading, the system cannot properly protect itself from overheating conditions. The unit may restrict operation automatically, but powering it through an active fault could risk further component damage.

A power reset may temporarily clear the fault display, but if the underlying sensor or wiring issue is still present, the E121 code will return once the system runs its diagnostics again. A reset is not a fix. The sensor connector, wiring harness, and sensor resistance all need to be physically checked and confirmed good before the error will stay cleared.

The first step, checking whether the sensor connector has separated from the PCB, is something a careful DIYer can perform after disconnecting power to the unit. However, accurately measuring sensor resistance and diagnosing faults in the wiring harness or PCB requires a multimeter and some familiarity with reading resistance values. If reseating the connector does not resolve the fault, professional diagnosis is recommended to avoid replacing parts unnecessarily.

Repair costs depend on which component has failed. If only the connector needs reseating, there may be no parts cost, just a diagnostic service call ranging from around $75 to $150. A replacement MCU temperature sensor typically costs in the range of $20 to $80 for the part itself. If the main control board requires replacement, total repair costs including labor can range from $300 to $700 or more depending on the model.