Lennox Furnace Error Code E405

E405 indicates that the Lennox System Operations Monitor detected an open circuit at the compressor, meaning the compressor lost its electrical path and could not run. This is typically caused by a power disconnection, a tripped breaker, or a blown fuse rather than a failed compressor. The fault is logged automatically when the compressor fails to complete a normal run cycle. Once the power issue is resolved and the compressor runs one full cycle, the error clears on its own.

A manual reset of the control board is not required to clear E405. The fault clears automatically after the compressor successfully completes one normal run cycle. However, if the underlying cause, such as a blown fuse or tripped breaker, has not been corrected, the error will return because the compressor still cannot operate.

If the cause is simply a tripped breaker, there is no parts cost involved. Replacing fuses at the disconnect typically costs between $10 and $50 for parts. If the fault is traced to a failed contactor or damaged wiring, repair costs including labor can range from $150 to $400 depending on the component and local service rates.

Start by checking the circuit breaker dedicated to the outdoor unit at your main electrical panel and reset it if tripped. Next, inspect the disconnect box near the outdoor unit for blown fuses, replacing any that test open with a multimeter. Once power is restored to the compressor circuit, allow the system to run through a complete cooling cycle, which will clear the fault automatically.

E405 itself is not a safety hazard, but working on the electrical components involved does carry risk. Always shut off power at the disconnect and breaker before inspecting fuses, contactors, or wiring at the outdoor unit. If you are not comfortable working with line-voltage electrical components, contact a qualified HVAC technician.