LG Air Conditioner Error Code E399

E399 indicates a heat sink temperature sensor error on the outdoor Fan2 PCB. The control system has detected that the sensor monitoring the Fan2 circuit board heat sink is producing a signal outside its expected range. This could point to a faulty thermistor, a wiring problem, or a damaged PCB. The system triggers this code to protect the fan driver components from potential overheating damage.

A power reset may temporarily clear the E399 code if the fault was caused by a transient condition such as a momentary poor connection. However, if the thermistor, wiring harness, or PCB has an underlying defect, the code will return after the unit restarts and begins monitoring the sensor again. A reset without diagnosis and repair is not a permanent solution.

Repair costs depend on what component has failed. Thermistor replacement is typically the least expensive path, with parts and labor costs generally ranging from around $75 to $200. If the Fan2 PCB itself needs replacement, total costs including parts and labor can range from approximately $250 to $600 or more depending on the specific unit model and local labor rates. Getting a diagnostic visit first will clarify which component is at fault before committing to a repair cost.

Some basic checks, such as inspecting for visible wiring damage or reseating a loose connector plug, are within reach of a knowledgeable DIYer comfortable working around outdoor AC components. However, properly diagnosing the thermistor requires a multimeter to measure resistance values, and replacing the Fan2 PCB involves handling refrigerant-adjacent electrical systems under AC power conditions. For most homeowners, a qualified HVAC technician is the safer choice.

Running the unit with an active E399 code is not recommended. The heat sink temperature sensor exists to protect the Fan2 PCB components from overheating, and operating without a reliable sensor reading removes that protection. Continued operation could lead to overheating of the fan driver circuitry and more extensive PCB damage. It is best to address the fault before resuming normal use.