Lennox: iComfort Communication Fault

What this fault means

On communicating Lennox systems, the furnace, thermostat and outdoor unit talk over a data bus; a communication fault disables staging or the whole system.

Likely causes

• Wiring fault on the communication bus
• Failed control or thermostat
• Power/grounding issue

What you can safely check

• Replace a dirty air filter — restricted airflow causes a surprising share of furnace faults.
• Confirm registers and the outdoor unit are clear of snow, leaves and debris.
• Cycle power at the switch once and let the system retry.

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