Lennox E120 Error Code

What does Lennox E120 mean?

E120 indicates that a Lennox communicating system component is responding more slowly than expected to thermostat queries. This is typically a transient condition that clears on its own and does not cause operational disruption. It is most often caused by a delay in the outdoor unit responding to the thermostat, or by inductive voltage on unused thermostat wire conductors coupling into the communication bus.

Symptoms

• E120 or 'Unresponsive Device' displayed on the thermostat, often briefly.
• System typically continues to operate normally — no hard shutdown.
• Alert clears automatically when the component responds to the next inquiry.
• Repeated E120 occurrences may indicate an underlying wiring issue.

Common causes

• Outdoor unit taking longer than expected to respond to thermostat communication — most common cause.
• Inductive voltage on unused thermostat wire conductors not tied to common.
• Marginal communication wiring with resistance or noise at the borderline of reliable communication.
• Firmware version mismatch between system components causing timing issues.

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Diagnostic steps

1. Observe frequency of E120 occurrences

   If E120 appears once and clears, no action is typically required. If it recurs frequently or accompanies E105, proceed with a wiring inspection. The alert is self-clearing when communication is restored.

2. Check unused thermostat wire conductors

   Land only the R wire on the R terminal to load the bundle with 24VAC. Then measure each remaining loose wire in AC mode against the C terminal. Voltage above 1.2VAC from inductive coupling on unused wires requires capping and connecting them to C.

3. Inspect communication wiring for marginal connections

   Tighten all terminal connections on the communication bus (i+ and i- terminals on the indoor, outdoor, and thermostat). A connection at the borderline of making contact causes intermittent delays that trigger E120.

4. Check for firmware compatibility between components

   If E120 appeared after adding a new accessory (Pure Air S, Apple Home Kit, etc.), verify the smart hub firmware version supports the added device. Per Lennox documentation, software version 3.4 or higher is required for Pure Air S and Apple Home Kit integration.

When to call a professional

If E120 recurs persistently despite wiring checks and inductive voltage correction, and particularly if accompanied by E105, upgrading to shielded communication wiring per Lennox Option 2 or 3 may be required. A licensed Lennox technician should also verify system firmware versions and recommission if components have been recently added.