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In my very recent experience with 2 Lennox units, both 15 years old, it could be a number of things. What you really need is a serviceman to diagnose the issue. Our problems were in sequence: the motherboard ($500.) (these things are very much related to computers), the gas valve ($400), or the hard to diagnose igniter. Over 2 years the serviceman replaced the first 2 and cleaned the igniter. They are very much not like the old gas furnaces many of us grew up with, which was basically a thermostat, burner, fan motor, gas valve, limit switch and anticipation switch.
In the last visit, the furnace "clicked" when the thermostat called for heat, but no joy. Turned out the igniter had "carboned over" and shorted it out.
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