LG Heat Pump Error Code E402

E402 indicates an outdoor temperature sensor breakaway, meaning the control board has lost the signal from the thermistor that monitors outdoor ambient temperature. This is typically caused by a disconnected connector, broken wire, or a failed sensor. The system relies on this sensor for proper operation, so the fault will usually interrupt normal heating or cooling function until it is resolved.

Running the system with an active E402 fault is not recommended. Without a valid outdoor temperature signal, the control board cannot make accurate decisions about defrost cycles, compressor staging, or capacity limits, which can lead to inefficient operation or potential component stress. It is best to address the fault before continuing regular use.

Repair costs vary depending on the root cause. If the fix is simply reseating a connector or cleaning corroded terminals, costs may be minimal beyond a service call fee, typically in the range of $75 to $150. Replacing a faulty outdoor temperature sensor usually costs between $50 and $150 for the part, plus labor. If the control board is involved, total repair costs can range from $300 to $600 or more depending on the specific board and labor rates in your area.

A homeowner or experienced DIYer can safely inspect the outdoor unit's sensor wiring and connectors for obvious disconnections, corrosion, or physical damage without specialized tools. However, measuring thermistor resistance and tracing wiring faults requires a multimeter and some technical comfort. Replacing the sensor or repairing internal wiring is a moderate-difficulty repair, and complex faults involving the control board should be handled by a qualified technician.

A power reset may temporarily clear the displayed code, but if the underlying sensor fault is still present, the E402 error will return once the system reinitializes and checks the sensor input. A reset is useful for confirming whether the fault is intermittent, but it does not repair a broken wire, failed thermistor, or corroded connector.