Why is my furnace short cycling?

A restricted filter is the simplest and most common cause — check and replace it before anything else, since a clogged filter starves the system of airflow and can trigger a safety shutoff that looks like short cycling. If the filter's clean, the next suspects are a flame sensor that's dirty or failing (causing the system to shut down prematurely as a safety measure) or a furnace that was oversized for the house, which naturally reaches temperature and shuts off faster than it should.

Short cycling isn't just an efficiency annoyance — the repeated start-stop cycle puts more wear on the ignition system and blower motor than normal operation, so a furnace doing this regularly is aging faster than it needs to.