Furnace starts, stops, starts again. That is short cycling.

What is actually happening

A normal furnace cycle runs 10 to 15 minutes. Short cycling is start, run a minute or two, quit, repeat. The furnace is not broken enough to stop — but something is tripping a safety limit or confusing a sensor, and every restart is wear on the igniter, the blower, and the gas valve.

The free check first: the filter

A completely clogged filter chokes airflow so badly the heat exchanger overheats and trips the limit switch — furnace shuts off, cools down, tries again. This is the single most common short-cycling call we get, and it costs nothing to check. Hold the filter up to a light: cannot see through it, it was due.

The usual mechanical suspects

• Flame sensor dirty or failing — the furnace lights, the sensor does not confirm flame, the gas valve shuts it down a few seconds later. Classic start-stop pattern.
• Blocked exhaust vent — a bird nest or ice at the vent termination trips the pressure switch. Check the outside vent after a storm.
• Blower or airflow problem — weak blower, crushed duct, closed registers — the exchanger overheats the same way a clogged filter causes.
• Oversized furnace — heats the space too fast, thermostat satisfies, shuts down, repeats. Cannot fix with a part; zoning or equipment work is the answer.
• Thermostat placement — a stat on a cold exterior wall or in a draft calls for heat when the house does not need it, then overshoots.

Why you should not just live with it

Every cycle is a thermal stress event — heat exchangers crack from expansion and contraction, and igniters are rated for a finite number of starts. A furnace that short cycles through a PA winter is spending its igniter budget and working toward a mid-January failure. The fix is almost always cheaper now than the no-heat call later.