Lennox EL296UHV Error Code E 105: Device Communication Problem

What Does Code E 105 Mean?

The Lennox EL296UHV uses a SureLight integrated control that is a communicating board: instead of simple on/off thermostat wires, it exchanges digital messages over an RS-Bus that links the icomfort thermostat, the indoor furnace, and the outdoor air conditioner or heat pump. Code E105 means the control counted numerous message errors and effectively found no other devices talking on that bus. Because this two-stage furnace coordinates its heat stages and variable-speed ECM blower over the bus, losing communication typically stops normal operation.

In most cases E105 is caused by electrical noise rather than a broken wire. When the low-voltage RS-Bus conductors are bundled or run in the same conduit as 120V or 240V power wiring, the high-voltage side can induce interference that corrupts the digital signal. Keeping the communicating wires physically separated from line-voltage power is the manufacturer's stated precaution.

The other primary cause is a loose or mis-wired connection at any of the three devices. A terminal that is not fully seated, a nicked conductor, or a wire moved onto the wrong terminal during service can drop the whole bus. Related codes help narrow this down: E120 flags one specific device that was slow to respond, E124 flags the icomfort thermostat's heartbeat going missing for more than three minutes, and E126 points at a hardware fault inside the control itself. E105 is the broad "no one is on the bus" symptom rather than a single-device fault.

E105 is self-clearing: once stable communication is restored across all devices, the fault goes away on its own.

What You'll Notice

• The icomfort thermostat shows a lost or no-connection message and will not command heat
• Heating and cooling stop entirely, or the system runs only intermittently and drops out
• The furnace does not respond to a call for heat even though it has power
• "E 105" appears on the control board's seven-segment LED display
• The problem began shortly after electrical work, a thermostat swap, or new wiring near the system

Common Causes

How This Is Diagnosed

A technician starts by confirming the RS-Bus wiring itself: correct polarity and landing at the thermostat, the furnace control, and the outdoor unit, with each terminal fully seated and no nicked or pinched conductors. They ohm the communicating wires end to end to rule out a break or a short and reseat the connectors after removing power.

If the wiring checks out, attention turns to noise. The technician looks for line-voltage cables, transformers, or other electrical noise sources routed close to the bus and re-routes the communicating wires away from them. Because E105 clears once the bus is stable, they then power the system back up and watch whether the thermostat and outdoor unit rejoin the network and the code stays gone.

When to Call a Professional

This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:

• E105 keeps returning and the thermostat repeatedly loses connection to the equipment
• The system only works intermittently, with heating cutting in and out
• The communication problem started after recent electrical, thermostat, or wiring work
• Other communicating codes such as E120, E124, or E126 appear alongside E105

✓ Verified against manufacturer service manual — March 2026