The component that fails first on a residential furnace is almost invariably the igniter (or flame sensor), followed closely by the blower motor run capacitor and the inducer draft motor. Hot-surface igniters glow intensely every time the furnace cycles on to light the burners, subjecting them to extreme thermal expansion and contraction that causes them to become brittle and burn out after several years of service. Similarly, flame sensors accumulate carbon soot over time, causing safety switches to shut the furnace down prematurely until cleaned or replaced.
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