Lennox Furnace Error Code E305

E305 on a Lennox furnace indicates that the LSOM (Lennox Smart Operation Module) detected an open start circuit for the compressor. This means the electrical path required to initiate compressor startup is broken or incomplete. The fault is typically caused by a failed start capacitor, an open compressor start winding, a faulty relay, or a wiring problem in the start circuit.

No, you should not attempt to force the system to run while E305 is active. An open start circuit means the compressor cannot start properly, and repeated failed start attempts can overheat the compressor motor windings and cause permanent damage. Leave the system off until the fault is diagnosed and repaired.

Repair costs for an E305 fault vary depending on the failed component. Replacing a start capacitor is typically the least expensive repair, often in the range of $75 to $200 including labor. A failed start relay or contactor may run $150 to $400. If the compressor itself has an open winding, replacement costs can range from $800 to $2,500 or more depending on the unit size and refrigerant type.

Diagnosing and repairing E305 requires working with high-voltage compressor circuits, which carry significant shock and injury risk. A technician will need to test capacitance values, measure winding resistance, and check relay operation under power. While a homeowner can visually inspect wiring for obvious damage, the actual repair and component testing should be performed by a qualified HVAC technician.

A reset may temporarily clear the displayed fault, but if the underlying start circuit problem has not been corrected, the E305 code will return as soon as the system attempts to start the compressor again. Resetting without repair does not fix the open circuit condition and may lead to further component damage.