Lennox Air Conditioner Error Code E423

E423 means the inverter module in your Lennox outdoor unit has detected an internal circuit problem, identified internally as inverter code 40. The inverter controls compressor speed and shuts down when it detects a fault to protect the system. This code can result from a failed inverter, power supply issues, overheating, or poor electrical connections. A technician should inspect the inverter and its power supply to determine the root cause.

The unit is designed to shut itself down when E423 is triggered, so continued operation is automatically prevented. Attempting to repeatedly reset and force the unit to run with an active inverter fault risks permanent damage to the inverter or the compressor. It is best to leave the unit off and schedule a technician inspection promptly.

Repair costs vary significantly depending on the root cause. If the issue is simply a loose connection or a one-time power surge, costs may be limited to a diagnostic service call fee, typically in the range of $75 to $150. If the inverter module itself requires replacement, parts and labor costs can range from $400 to over $1,200 depending on the specific unit model and regional labor rates. A technician diagnosis is necessary before accurate cost estimates can be provided.

The first step you can safely take is disconnecting power to the outdoor unit, waiting a few minutes, and restoring power to see if the error clears. However, if the fault returns, diagnosing and repairing inverter circuit problems involves high-voltage components and requires a qualified HVAC technician. Attempting to test or replace the inverter without proper training and equipment is not recommended.

A power reset by disconnecting the outdoor unit and restoring it is the recommended first diagnostic step and may clear the error if it was caused by a transient power disturbance. However, if the fault is due to a hardware problem such as a failing inverter or wiring issue, the code will return after reset. Importantly, if the error triggers 10 times within one hour, the control locks out the unit and a power reset becomes required just to restore basic operation.