LG Air Conditioner Error Code C4

The C4 code on an LG air conditioner signals a heat sink sensor error in the outdoor unit. It means the heat sink thermistor is either open-circuited, short-circuited, or the heat sink temperature has exceeded 95 degrees Celsius. The system shuts down automatically to prevent damage to the power electronics. A technician should check the thermistor connection at CN-TH2 on the outdoor PCB and measure the thermistor resistance to begin diagnosing the fault.

No, you should not attempt to bypass or ignore the C4 error. The code indicates either a failed sensor or a heat sink temperature that has exceeded 95 degrees Celsius. Continued operation under true overtemperature conditions can damage the inverter power module, leading to a much more expensive repair. The system shutting down is a protective function.

Repair costs vary depending on the root cause. Cleaning a blocked outdoor unit is inexpensive and may only involve a service call fee, typically in the range of $75 to $150. Replacing the heat sink thermistor is a relatively low-cost part repair, often in the $100 to $250 range including labor. If the outdoor control PCB requires replacement, costs can range from $300 to $600 or more depending on the unit model and parts availability.

Some basic checks are within reach of a careful DIYer, such as inspecting the outdoor unit for blocked airflow, cleaning debris from the fins, and visually confirming that no wiring is visibly disconnected. However, measuring thermistor resistance and accessing the outdoor PCB connector CN-TH2 requires opening the outdoor unit and using a multimeter, which carries electrical risk if not done properly. Replacing the thermistor or PCB is best handled by a qualified technician.

A power reset may temporarily clear the code and allow the unit to restart, but the error will return if the underlying cause has not been fixed. If the thermistor is faulty or the heat sink is overheating due to blocked airflow or a fan problem, the C4 code will reappear, often quickly. Resetting without diagnosis is not a solution.