Lennox Furnace Error Code E408

E408 on a Lennox furnace indicates that the LSOM module has detected a welded compressor contactor, meaning the compressor is running continuously and cannot be shut off through normal control signals. The contactor is a high-voltage switching component that controls power to the compressor. When its internal contacts fuse together, the compressor circuit stays closed regardless of control commands. This fault requires inspection of the contactor to determine whether the contacts have physically failed.

Running the system with a welded contactor is not advisable for extended periods. A compressor that cannot shut off will overheat, which can lead to compressor damage or failure. You should have the contactor inspected by a qualified technician before continuing normal system operation, especially if the fault code returns after a reset.

Replacing a compressor contactor involves working with high-voltage components, typically 240 volts, on the outdoor unit. This work carries a serious risk of electrical shock and should only be performed by a qualified HVAC technician or licensed electrician. In many areas, HVAC electrical repairs also require proper licensing and permitting, so professional service is strongly recommended.

A power reset or one normal compressor run cycle can clear the E408 fault code from the display. However, if the contactor contacts are physically welded, the underlying problem will remain and the code will return once the system restarts. Clearing the code is only a permanent fix if the fault was caused by a one-time communication timing issue rather than an actual welded contactor.

The cost to replace a compressor contactor typically falls in the range of $150 to $400, depending on labor rates in your area and the specific contactor required for your unit. If the welded contactor caused secondary damage to the compressor itself, repair costs can be significantly higher. Getting a diagnosis from a licensed HVAC technician before committing to repairs is recommended.