Lennox Furnace Error Code E404

E404 is a LSOM fault indicating the compressor has experienced a locked rotor condition, meaning the compressor motor failed to spin when power was applied. Common causes include a shorted run capacitor, seized internal bearings, or excessive liquid refrigerant in the compressor. The system will attempt to recover automatically after 4 consecutive normal run cycles or after a power reset.

Continuing to attempt operation with a locked rotor condition can cause significant heat buildup in the compressor windings and accelerate internal damage. If the error clears and the system runs normally, monitoring it closely is reasonable in the short term. If the fault repeats, the system should be shut down and inspected by a qualified technician to prevent compressor failure.

The run capacitor is the first component to test, since a short or failure there is a common and straightforward cause of the locked rotor fault. After confirming capacitor condition, the technician should check supply voltage at the compressor terminals during a start attempt, inspect the contactor, and evaluate refrigerant charge and system pressures. If all external components test correctly and the fault persists, internal compressor failure should be considered.

A power reset can clear the E404 fault, and the system is also designed to clear it automatically after 4 consecutive normal run cycles complete without a recurrence. However, if the underlying cause has not been addressed, the fault will return shortly after reset. A persistent or repeating E404 requires hands-on diagnosis of the run capacitor, refrigerant system, and compressor.

Repair costs vary widely depending on the root cause. Replacing a run capacitor is generally the least expensive fix, typically ranging from around $75 to $250 including parts and labor. Compressor replacement is significantly more involved and can range from $1,000 to $2,500 or more depending on the unit and refrigerant type. Refrigerant-related corrections fall somewhere in between and depend on the extent of the issue.