Thermostat displays E351; furnace may shut down unexpectedly or fail to start a heating cycle.

Safety: Turn off power and allow the furnace to cool before inspection.

• 1. Replace or clean filter: Remove the blower door and slide out the Lennox-approved air filter. Replace if dirty or clogged. Use the correct size and MERV rating recommended by Lennox.
• 2. Check airflow: Open all supply and return registers. Remove any obstructions such as furniture or rugs blocking airflow.
• 3. Inspect blower operation: With power restored, call for heat and observe if the indoor blower starts when expected. If the motor hums or fails to start, it may be failing.
• 4. Reset lockout: After correcting airflow issues, power the furnace OFF for 30 seconds, then ON. Monitor a full heating cycle to ensure the limit does not trip again.

If E351 recurs, a technician should check static pressure, duct sizing, and the high limit switch.