LENNOX ELITE FURNACE SHORT CYCLING General Troubleshooting

High-Efficiency Furnace Air Filter (MERV 11)

The most frequent cause of short cycling is restricted airflow due to a loaded filter. This causes the internal cabinet temperature to rise rapidly, triggering the high-limit safety switch to break the 24V circuit to the gas valve. Replacing the filter restores proper CFM across the heat exchanger, keeping temperatures below the trip threshold.

Flame Sensor

Lennox units often experience carbon or silica buildup on the flame sensor rod, which prevents the 'flame rectification' process. If the control board does not detect a steady microamp signal within seconds of ignition, it will shut off the gas valve for safety. A new sensor ensures the control board accurately detects the burner flame.

SKU: 71W80

Primary High Limit Switch

If airflow is confirmed to be adequate but the unit continues to cycle, the high-limit switch may have become 'weak' due to repeated thermal cycling or mechanical fatigue. This bimetallic disc switch is designed to fail open for safety; if it trips prematurely or fails to reset properly, the furnace will cycle regardless of actual heat exchanger temperature.