LG Heat Pump Error Code CH40

CH40 on an LG heat pump indicates a problem with the CT (current transformer) sensor circuit. This includes an initial current error at startup or an open or short condition within the current detection circuit. The system uses the CT sensor to monitor AC input current, and when it cannot get a valid reading, it stops operation and displays this code. Diagnosing the fault requires checking input voltage, the C01N signal voltage, and the current transformer component.

You should not attempt to bypass or ignore the CH40 error. The CT sensor circuit monitors the electrical current drawn by the unit, and a malfunction in this circuit means the system cannot verify safe operating current levels. Continued operation without a functioning current monitoring circuit could allow electrical faults to go undetected. The unit should remain off until the fault is diagnosed and repaired.

Checking the AC input voltage with a multimeter is something a knowledgeable DIYer can do safely when following proper electrical precautions. However, measuring the C01N signal voltage at the PCB, inspecting the current transformer, and evaluating the main PCB require working with live high-voltage circuits and familiarity with control board diagnostics. For most homeowners, this repair should be handled by a qualified HVAC technician.

Power cycling the unit may temporarily clear the CH40 code, but if the underlying fault in the CT sensor circuit has not been corrected, the error will return on the next startup. A reset is useful only for confirming whether the fault is intermittent. Persistent CH40 errors require proper electrical diagnosis of the input power, CT sensor, and main PCB.

Repair costs vary depending on the root cause. If the issue is simply an unstable power supply or a wiring problem, labor costs for diagnosis and repair could range from around $80 to $200. If the current transformer component needs replacement, parts and labor combined may range from $150 to $350. A main PCB replacement is typically the most expensive outcome, often ranging from $300 to $700 or more depending on the specific model.