LG HVAC Error Code CO

The CO code means the indoor unit's main PCB has detected that the air thermistor circuit is open or shorted. The thermistor is the sensor that monitors return air temperature inside the unit. The code points to either a failed thermistor, a wiring or connector problem, or in some cases a faulty PCB.

The system may shut down or refuse to operate normally when the CO code is active because the PCB cannot reliably read air temperature. Running the unit in this condition risks improper temperature control and potential compressor stress. It is best to diagnose and resolve the fault before continuing regular operation.

A power reset may temporarily clear the code, but it will return as soon as the PCB polls the thermistor circuit again and detects the same open or shorted condition. The underlying hardware fault must be corrected before the error will stay cleared. Resetting without repairing is not a solution.

A knowledgeable DIYer can perform the thermistor resistance test by submerging the sensor in water at a known temperature and measuring resistance with a multimeter. The target values are 20,700 ohms at 10°C (50°F) and 5,600 ohms at 38°C (100°F). Connector inspection and reseating is also a straightforward check. However, if the PCB needs replacement, that repair is best handled by a qualified technician.

If the thermistor is the cause, sensor replacement typically costs between $50 and $150 including parts and labor. If the PCB is at fault, repair costs generally range from $200 to $600 or more depending on the unit model and labor rates in your area. Getting a diagnosis first will help clarify which component needs replacement before committing to a repair cost.