Lennox Furnace Error Code E105

E105 means the furnace has detected a communication problem on the RS Bus network and cannot find any other devices, such as the thermostat or outdoor unit, communicating on that network. The RS Bus is a dedicated low-voltage serial communication system that connects all components in a Lennox communicating system. This error typically points to a wiring fault, loose connection, or interference issue rather than a component failure.

An E105 fault indicates the furnace has lost communication with the rest of the system, which typically prevents normal operation until the issue is resolved. Running the furnace in this state is not recommended because the control system cannot properly coordinate heating or cooling cycles with other connected equipment. Address the fault before resuming regular use.

If the problem is limited to a loose or miswired connection, repair costs would primarily cover a technician's diagnostic and labor time, typically in the range of $75 to $200 depending on your area. If a control board with a failed communication circuit needs replacement, parts and labor combined can range from $300 to $700 or more depending on the specific board and the unit involved. Getting a diagnosis first is important because many E105 faults are resolved without any parts replacement.

Some causes of E105 are accessible to a knowledgeable homeowner, such as checking for loose or disconnected wires at the thermostat or visible terminal boards. However, inspecting wiring inside the furnace cabinet and outdoor unit involves working near high-voltage components and should be handled by a qualified technician. Diagnosing electrical noise sources and confirming correct RS Bus wiring also benefits from professional experience with communicating HVAC systems.

Resetting the furnace may temporarily clear the error code, but if the underlying wiring or communication problem has not been corrected, the code will return. A reset does not fix loose connections, miswired terminals, or noise interference. The root cause must be identified and corrected for the error to clear permanently.