Furnace short cycling (turns on and off every few minutes): causes and fixes

Short bursts of heat followed by shutdown, over and over. Hard on the furnace, expensive on the bill, and almost always one of these.

Clogged filter

The number one cause. Restricted airflow overheats the furnace in minutes, the high-limit switch cuts the burners, the furnace cools, relights, overheats again. Replace the filter first, always.

Blocked or closed vents

More than a couple of closed supply registers does the same thing as a dirty filter. Open them, including in unused rooms, and make sure furniture is not sitting on returns.

Oversized furnace

If the furnace has short-cycled since the day it was installed, it may simply be too big for the house: it satisfies the thermostat in minutes and shuts down. There is no cheap fix; a two-stage retrofit or right-sized replacement is a contractor conversation.

Thermostat placement or fault

A thermostat above a supply register or in direct sun reads warm too early and kills the cycle. Check what is blowing on it. Swapping in a $25 thermostat is a valid diagnostic.

Flame sensor dropout

If each "cycle" is only seconds long and you hear repeated ignition clicks, the flame sensor is failing to prove flame. The board will usually flash a code — look it up here to confirm before calling.

Flue or intake blockage

High-efficiency furnaces with PVC intake/exhaust through the wall: snow, leaves, or a nest blocking the pipe trips the pressure switch. Check both pipe terminations outside.

Safety line: Short cycling caused by overheating bakes the heat exchanger over time. A season of ignoring it can turn a $20 filter into a $2,000 repair. If a clean filter and open vents do not fix it within a day, book a technician.

Is the furnace flashing a light?

If the burners will not light, the control board is almost certainly flashing a diagnostic code behind the little window. Count the flashes and look the code up by brand here — walking into the phone call knowing the code saves diagnostic time.

Error codes behind this symptom

If your furnace is flashing a code while doing this, these code families are the usual suspects: limit switch codes, pressure switch codes, flame sensor codes. Look your exact code up from the brand index, and see the furnace repair cost guide for what the fix typically runs.