Lennox Furnace Error Code E407

E407 indicates that the LSOM (Load Shedding Output Module) detected an open run circuit at the compressor. The control system monitors current through a run current transformer and found that the required current level was not present. This fault protects the system from continued operation when the compressor circuit is not functioning correctly. It can be caused by a failed capacitor, open compressor winding, faulty wiring, or a defective current transformer.

It is not advisable to continue operating the system with an active E407 fault. The error indicates the compressor is not functioning correctly, and continued attempts to run it without diagnosing the cause can lead to more serious and expensive damage. The system should be inspected by a qualified technician before resuming normal operation.

Knowledgeable DIYers can perform some initial checks, such as inspecting visible wiring for loose or burned connections and visually inspecting the run capacitor for signs of bulging or leaking. However, testing compressor winding resistance, measuring capacitor values, and checking current transformer output requires proper tools and electrical safety knowledge. Work inside the electrical cabinet involves line voltage and should be approached with caution. When in doubt, a licensed HVAC technician should perform the diagnosis.

A full power reset is one of three conditions that can clear E407. The fault will also clear if current is successfully sensed by the RUN sensor or if one normal compressor run cycle completes. However, if the underlying electrical fault is still present, the code will return. A power reset should only be used as a diagnostic step, not as a long-term fix.

Repair costs vary significantly depending on the root cause. Replacing a run capacitor is typically the least expensive repair, often ranging from $75 to $250 including labor. Wiring repairs generally fall in a similar range. Replacing a failed compressor is the most costly outcome, potentially ranging from $800 to $2,500 or more depending on the unit and labor rates in your area.