The furnace starts in a sequence, and clicking without heat means one step failed: the inducer proves the vent, the igniter glows or sparks, the gas valve opens, and the flame sensor proves flame. A sensor filmed with oxide, a cracked igniter, and a failing inducer motor are the common findings, roughly in that order, and each is a part level repair. What it is not, usually, is the thermostat. If you smell gas at any point, stop: leave, and call the gas utility or emergency services before any repair call.
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