LG Heat Pump Error Code E309

E309 indicates an OPEN or SHORT fault on the discharge temperature sensor for fixed capacity compressor 2 in the outdoor unit. The system detects this fault when the outdoor temperature is above -10°C and the sensor voltage falls outside the valid range: above 4.95V for an open circuit or below 0.5V for a short circuit. The error is designed to protect the compressor from running without reliable temperature monitoring.

It is not advisable to continue running the heat pump while E309 is active. The discharge sensor protects compressor 2 from overheating, and without a valid sensor signal, the control system cannot monitor compressor discharge temperatures. Continued operation could result in compressor damage, which is a significantly more expensive repair than fixing the sensor itself.

A power reset may temporarily clear the error code from the display, but the fault will return as soon as the system resumes operation if the underlying sensor or wiring problem has not been fixed. Because the error is only detected when outdoor temperatures are above -10°C, a reset during cold conditions might delay the reappearance of the code. A proper repair of the sensor circuit is required for a permanent resolution.

Some steps, such as inspecting the wiring harness for visible damage and checking that sensor connectors are fully seated and free of corrosion, can be done by a knowledgeable DIYer. However, actually testing the thermistor resistance, accessing the outdoor control board, and replacing refrigerant-side components requires tools and expertise that typically call for a qualified HVAC technician. Working inside the outdoor unit also involves electrical hazards that should be approached with caution.

Repair costs depend on the root cause. Replacing a discharge thermistor sensor typically ranges from $75 to $250 including parts and labor. If the wiring harness needs repair or replacement, costs may rise to $150 to $400. A faulty outdoor control board is the most expensive scenario, often ranging from $400 to $900 or more depending on the unit model and labor rates in your area.