LG Air Conditioner Error Code 40

Error code 40 indicates a fault with the inverter compressor CT sensor in the outdoor unit. The system's Micom control board checks the CT sensor input voltage at the moment power is applied and expects a reading of 2.5V plus or minus 0.3V. When that voltage is outside the 2.2V to 2.8V range, the unit shuts down and displays this code. Causes include abnormal supply voltage, a failed CT sensor, damaged wiring, or a faulty inverter board.

No. When error code 40 is active, the system prevents operation specifically to protect the compressor and inverter components from damage caused by unmonitored current conditions. Attempting to force the unit to run while this fault is present could result in compressor damage or electrical hazards. The fault should be diagnosed and resolved before resuming normal operation.

Repair costs vary depending on the root cause. If the issue is simply an electrical supply problem corrected at the breaker or panel, costs may be limited to a service call fee, typically in the range of $75 to $150. CT sensor replacement parts and labor can run from $150 to $350 depending on the model and your location. An inverter control board replacement is more involved, often ranging from $300 to $600 or more for parts and labor combined.

The first step, checking the L1-L2 supply voltage at the outdoor unit terminal block, can be done by a confident DIYer with a multimeter and appropriate safety precautions. However, diagnosing and replacing the CT sensor or inverter control board involves working inside the outdoor unit with high-voltage components and should be handled by a qualified technician. Incorrect handling of inverter circuitry can cause serious injury.

A power reset may temporarily clear the code, but if the underlying cause, such as a faulty CT sensor, wiring fault, or supply voltage problem, has not been corrected, the error will return immediately or shortly after the next startup. The Micom checks the CT sensor voltage during every initial power-up sequence, so the fault will reappear as long as the root cause remains.