Lennox Furnace Error Code E403

E403 is an LSOM fault indicating the outdoor unit compressor is short-cycling, running for less than 4 minutes before shutting down due to a pressure trip. The pressure trip can come from either the high-pressure or low-pressure safety switch in the outdoor unit. The code is the system's way of flagging a repeated pattern of premature compressor shutdowns that needs to be diagnosed. A technician should check refrigerant pressures, condenser airflow, and fan operation to identify the root cause.

Continuing to operate the system without addressing E403 is not recommended. The pressure safety switches are protecting the compressor from damage caused by abnormal operating conditions. Repeated short-cycling under pressure stress can shorten compressor life significantly. It is best to have the system inspected before resuming normal operation.

The first step is to connect manifold gauges to the system and record both high-side and low-side pressures during operation to determine whether a high-pressure or low-pressure trip is occurring. From there, the technician should inspect the condenser coil for blockage, verify the condenser fan is operating correctly, and assess refrigerant charge. Identifying which pressure switch is tripping narrows the diagnosis significantly and guides the next steps.

Yes, a power reset will clear the E403 code immediately. However, the code will return if the underlying cause of the pressure trip is not corrected. The system will also clear the fault automatically after 4 consecutive normal compressor run cycles of at least 4 minutes each, without a manual reset.

Repair costs vary widely depending on the root cause. Condenser coil cleaning is typically a lower-cost service call in the range of $100 to $250. Refrigerant leak diagnosis, repair, and recharge can range from $250 to $800 or more depending on the extent of the leak. A condenser fan motor replacement generally falls in the $300 to $600 range including labor. Compressor replacement, if the short-cycling has caused compressor damage, can run $1,200 to $2,500 or higher.