LG Heat Pump Error Code E109

E109 on an LG heat pump indicates a fault with auxiliary heater sensor 2, which is a temperature sensor located in the electric backup heater assembly. The control board is receiving a signal from this sensor that is outside the normal operating range, or it is receiving no signal at all. This typically points to a failed sensor, wiring issue, or connector problem.

The system may limit or shut down the auxiliary heater when E109 is active, which is a protective response. Running the heat pump in this condition in mild weather is generally low risk, but in very cold temperatures the reduced heating capacity could be a problem. It is best to have the fault diagnosed and repaired promptly rather than leaving it unaddressed.

A reset may temporarily clear the E109 code, but if the underlying problem with the sensor or its wiring is still present, the error will return once the system attempts to use or monitor the auxiliary heater again. A reset is useful for confirming whether the fault is intermittent, but it is not a substitute for properly diagnosing and repairing the failed component.

Some basic checks, such as inspecting the sensor wiring connector for looseness or corrosion, can be done by a knowledgeable DIYer. However, working near the auxiliary electric heater circuit involves components that carry line voltage, so any hands-on repair beyond visual inspection should be performed by a qualified HVAC technician. Replacing the sensor requires accessing internal components and matching the correct thermistor type.

Repair costs depend on the root cause. A sensor replacement is typically an inexpensive part, so the cost is mostly labor, often in the range of $100 to $250 for a service call and repair. If the wiring harness or control board needs replacement, costs can rise to $300 to $700 or more depending on parts pricing and labor rates in your area.