Lennox E105 Error Code

What does Lennox E105 mean?

E105 is a communication problem alert on Lennox communicating systems, indicating that one or more system components cannot reliably communicate over the communication bus. It is almost always caused by electrical noise from high-voltage wiring run too close to the low-voltage communication wires, improper grounding, or loose connections. The alert clears automatically when the system detects normal communication.

Symptoms

• E105 or 'Communication Problem' displayed on the Lennox thermostat.
• Intermittent loss of thermostat control — system may stop responding to demand changes.
• Other alert codes from specific components may appear alongside E105.
• Alert may appear after new accessories are added to the communication bus.

Common causes

• High-voltage wiring routed parallel to or bundled with low-voltage communication wires.
• Loose or poorly spliced terminal connections on the communication bus wiring.
• Improper or missing earth ground on the indoor unit or transformer.
• Inductive voltage above 1.2VAC on unused thermostat wires not capped and tied to common.
• Incompatible accessory firmware connected to the communication bus.

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Diagnostic steps

1. Verify high-voltage wiring is separated from low-voltage communication wiring

   Trace the thermostat wire from the indoor unit to each component. Confirm it is not bundled or run parallel with line-voltage wiring in wall, ceiling, or floor cavities. Separate them by at least 6 inches where possible.

2. Check and tighten all terminal connections

   Use a 3/32-inch slotted screwdriver to tighten all screw terminals on the indoor unit, outdoor unit, thermostat, and any accessories. Lennox specifically recommends this tip size to ensure proper seating without stripping.

3. Cap and tie unused thermostat wires to common

   All unused conductors in the thermostat cable must be capped and connected to the C (common) terminal on the indoor unit control board. Unconnected conductors act as antennas, picking up inductive voltage that disrupts communication.

4. Measure inductive voltage on each communication wire

   Set a voltmeter to AC mode and probe each wire against the C terminal. Voltage up to 0.3VAC is acceptable; 0.3–0.7VAC may work; above 1.2VAC must be resolved by re-routing wiring or switching to shielded cable.

5. Disconnect accessories one at a time if E105 persists

   Per Lennox troubleshooting procedure, disconnect all accessories except the thermostat-to-indoor-unit connection, then reconnect one device at a time and recommission. The device that reintroduces E105 is the source.

When to call a professional

If re-routing wiring, tightening connections, and capping unused wires do not resolve E105, replacing the low-voltage wiring with shielded 18-22 AWG cable is required per Lennox Option 2/3 wiring guidance. Running new wire through walls and reconnecting all system components is a task best performed by a licensed HVAC technician familiar with Lennox communicating system wiring requirements.