LG Heat Pump Error Code E409

E409 indicates a discharge temperature sensor error on fixed capacity compressor 2. The outdoor unit PCB has detected a signal voltage above 4.95V or below 0.5V from that sensor, which corresponds to temperatures outside the physically plausible range of -30°C to 151°C. This condition is only flagged when the outdoor temperature is above -10°C. The system restricts compressor 2 operation to prevent damage until the sensor fault is corrected.

Continuing to operate the unit with an active E409 fault is not recommended. The discharge temperature sensor is a primary protection device for compressor 2, and without a reliable reading the system cannot prevent the compressor from overheating. Running the unit in this condition risks compressor damage, which is a significantly more expensive repair than resolving the sensor fault.

Repair costs vary depending on the root cause. If only the discharge sensor thermistor needs replacement, parts and labor typically range from around $100 to $300. A wiring harness repair or connector replacement may fall in a similar range. If the fault has caused compressor damage or the PCB requires replacement, costs can rise to $400 to $900 or more depending on the specific unit and local labor rates.

A knowledgeable DIYer can perform initial checks such as verifying the sensor connector is fully seated on the PCB and visually inspecting the sensor harness for obvious damage. However, measuring thermistor resistance accurately, interpreting the results against a resistance-temperature curve, and replacing components in a refrigerant system requires proper tools and, in many jurisdictions, EPA certification for refrigerant handling. If basic connector checks do not clear the fault, a qualified technician should complete the diagnosis.

A power reset may temporarily clear the fault display, but the error will return if the underlying sensor or wiring problem has not been corrected. The PCB will re-evaluate the sensor voltage each time the system attempts to run compressor 2, and any reading outside the 0.5V to 4.95V range will re-trigger E409. A reset is useful only as a step to confirm whether the fault is intermittent after a repair has been made.