Lennox Furnace Error Code E401

E401 is a LSOM (Lennox System Operation Monitor) fault indicating the compressor ran for more than 18 hours continuously to satisfy a single thermostat demand without achieving the setpoint. The LSOM also monitors low pressure switch trips as part of this diagnostic. The fault signals a significant efficiency or refrigerant system problem that needs to be diagnosed before the system is returned to service.

Operating the system without addressing the root cause risks further wear on the compressor, which is already under stress from extended runtime. Continued operation with a low refrigerant charge can cause compressor overheating and eventual failure. It is best to have a technician diagnose and correct the issue before resuming normal use.

Repair costs depend entirely on the root cause. A simple filter replacement costs only a few dollars. Refrigerant leak detection and recharge typically runs between $200 and $600 or more depending on the extent of the leak and the refrigerant type. Low pressure switch replacement generally falls in the $100 to $300 range including labor. Compressor replacement is the most expensive scenario, often ranging from $1,200 to $2,500 or higher depending on the unit.

Yes, a power reset will clear the E401 code immediately. It will also clear automatically after 30 consecutive normal run cycles. However, if the underlying cause is not corrected, the fault will return once the compressor again exceeds the 18-hour runtime threshold during a single demand cycle.

Start by checking and replacing a dirty air filter, which is the simplest possible cause. Beyond that, a qualified technician should check refrigerant pressures at the service ports, inspect the low pressure switch for repeated tripping, evaluate evaporator coil condition, and assess compressor performance. If refrigerant is low, the leak must be located and repaired before recharging the system.