Lennox Air Conditioner Error Code E105

E105 means the outdoor control board has lost communication with either the thermostat or the indoor unit due to too many message errors on the RSBus communication network. The RSBus is the low-voltage serial data link that connects all three system components. This fault is most commonly caused by electrical noise from high-voltage wiring interfering with the RSBus signal, or by loose and mis-wired connections at one of the components.

A reset may temporarily clear the E105 fault, but if the underlying cause, such as electrical noise interference or a loose connection, is still present, the error will return quickly. Resetting without fixing the root cause is not a solution and can delay proper diagnosis. A thorough inspection of the RSBus wiring and all terminal connections is necessary to resolve the fault permanently.

If the problem is simply loose connections or improperly routed wiring, repair costs are primarily the cost of a service call, which typically ranges from around 75 to 150 dollars depending on your area. If a control board needs to be replaced, total costs including parts and labor generally range from 300 to 700 dollars or more depending on the specific board and local labor rates. Getting a diagnostic from a certified Lennox technician before authorizing parts replacement is recommended.

Start by checking that the RSBus low-voltage communication wiring is physically separated from any high-voltage power wiring throughout the entire wire run. Then inspect every wiring connection at the thermostat, indoor unit, and outdoor unit for loose terminals or incorrect wire placement. If the wiring checks out, the indoor or outdoor control board may need to be tested or replaced by a qualified technician.

The E105 fault itself is a communication error rather than a refrigerant or electrical safety hazard, but the system will not operate normally while the fault is active. The most relevant safety consideration is the possibility that high-voltage and low-voltage wiring have been improperly combined, which a qualified technician should inspect and correct. Do not attempt to access or rewire electrical components unless you are trained to work with HVAC electrical systems.