LG Heat Pump Error Code P781

P781 on an LG heat pump indicates that heating mode operation is being blocked because the outdoor ambient temperature has been detected as exceeding 30°C (86°F). This is a built-in operational restriction, not a component failure. The system uses the outdoor temperature sensor to enforce this limit automatically. If outdoor temperatures are actually cool and the code still appears, the sensor may be inaccurate.

P781 is not a safety hazard. It is a protective operational restriction that prevents the heat pump from running in heating mode when outdoor temperatures are too high for that mode to be appropriate or efficient. The unit will continue to operate normally in cooling mode or fan mode if needed. There is no risk of damage to the unit from this code appearing.

If the code is due to actual high outdoor temperatures, there is no repair cost at all. If a faulty outdoor ambient temperature sensor is the cause, sensor replacement parts typically range from $20 to $80 depending on the specific model, with professional labor adding $75 to $200 depending on your location and the technician's rates. Wiring repairs, if needed, are generally on the lower end of that labor range.

If outdoor temperatures are genuinely above 30°C, no fix is needed since the unit is behaving correctly and will resume heating once temperatures drop. If the code appears during cool weather, check the outdoor ambient temperature sensor for accuracy by comparing its reading to an external thermometer. Also inspect the sensor wiring for corrosion or loose connections. A faulty sensor will need to be replaced to resolve a false P781 condition.

Resetting the unit will not permanently clear P781 if the outdoor temperature is still above 30°C or if a faulty sensor is continuously reporting a temperature above that threshold. The restriction will reactivate as soon as the control system reads the temperature again. To permanently resolve the code in cool weather, the underlying sensor or wiring issue must be diagnosed and corrected.