LG Heat Pump Error Code E312

E312 indicates a fault with the Sump Temperature Fixed 1 sensor in the outdoor unit. The control board has detected that the sensor signal voltage is either above 4.95V or below 0.5V, which corresponds to an open or short circuit condition. This error only activates when the outdoor temperature is above -10°C. The sensor must be diagnosed and repaired before normal operation can resume.

Operating the heat pump while E312 is active is not recommended. The sump temperature sensor provides data the system uses for safe compressor operation, and running without that input could lead to compressor damage over time. It is best to address the fault promptly to avoid more costly repairs.

A power reset may temporarily clear the E312 error code, but it will return if the underlying sensor fault has not been repaired. Because the error is triggered by specific voltage thresholds on the sensor circuit, the system will detect the same out-of-range condition as soon as it runs its diagnostic checks again. A proper repair of the sensor or wiring is required for the fault to stay resolved.

A knowledgeable DIYer can inspect the sensor wiring and connectors for visible damage, corrosion, or loose connections without specialized tools. However, testing the thermistor resistance and tracing faults to the control board requires a multimeter and familiarity with heat pump wiring. Refrigerant-related work must be performed by a certified technician, though this error is sensor-specific and does not directly involve refrigerant handling.

Repair costs vary depending on the root cause. Replacing a failed thermistor sensor typically costs between $80 and $200 including parts and labor. If the control board needs replacement, costs can range from $300 to $600 or more. A diagnostic service call to identify the exact fault usually runs between $75 and $150.