Lennox Furnace Error Code E400

E400 on a Lennox system means the LSOM has detected that the compressor internal overload protector has tripped. This happens when the compressor overheats or draws excessive current, causing its built-in safety device to shut it down. The thermostat is sending a Y1 cooling demand, but the compressor will not run until the overload resets and the underlying cause is corrected.

The compressor will not run while the overload is tripped, so there is no immediate safety hazard in the home. However, repeatedly cycling the system while the fault is active puts additional stress on the compressor and can shorten its life. It is best to leave the system off until a technician can diagnose and correct the root cause.

Repair costs vary widely depending on the cause. Cleaning a condenser coil typically runs in the range of $75 to $200. Refrigerant leak repair and recharge can range from $200 to $600 or more depending on the extent of the leak. Compressor replacement is the most expensive outcome, generally falling in the range of $1,200 to $2,500 or higher depending on the unit model and labor rates in your area.

Some steps are safe for homeowners, such as checking that the outdoor disconnect and circuit breaker are on, clearing debris from around the outdoor unit, and cleaning a visibly dirty condenser coil. However, diagnosing refrigerant charge issues, testing compressor windings, and replacing components like contactors or the compressor itself require a licensed HVAC technician. Attempting refrigerant work without proper certification is illegal and dangerous.

The compressor overload protector will reset automatically once the compressor cools down, which can take 30 minutes to several hours depending on how hot it became. The E400 code may clear after a reset, but if the underlying problem such as a dirty coil, low refrigerant, or a failing compressor is not fixed, the overload will trip again and the code will return. A reset alone is not a repair.