Furnace troubleshooting
Furnace Short Cycling (Turning On and Off)? Here's Why
Short cycling — switching on and off rapidly — wastes fuel and wears out the furnace. It usually points to overheating or a sensor problem.
Common causes
Clogged air filter
You can checkRestricted airflow makes the furnace overheat and shut off on its limit switch, over and over.
Dirty flame sensor
Pro repairA sooty flame sensor can't confirm the flame, so the furnace shuts down seconds after lighting.
Blocked vents or flue
You can checkClosed registers or a blocked exhaust cause overheating and safety shutdowns.
Oversized furnace or bad limit switch
Pro repairAn oversized unit heats too fast; a failing limit switch shuts it down prematurely.
What you can check yourself
- Replace the air filter.
- Open all supply and return vents; don't close off rooms.
- Make sure the outdoor exhaust/intake isn't blocked by snow, leaves, or debris.
- Note how long it runs before cutting out — seconds points to flame sensor; minutes points to overheating.
When to call a pro
If a new filter and open vents don't fix it, the flame sensor or limit switch likely needs service before it damages the furnace.
Most furnace repairs run $150–$800 — see our repair cost guide.
Other furnace problems
FAQ
Furnace short cycling — questions
Short cycling — switching on and off rapidly — wastes fuel and wears out the furnace. It usually points to overheating or a sensor problem. The most common causes are clogged air filter, dirty flame sensor, blocked vents or flue.
You can safely check a few things first: Replace the air filter. Open all supply and return vents; don't close off rooms. Make sure the outdoor exhaust/intake isn't blocked by snow, leaves, or debris. If a new filter and open vents don't fix it, the flame sensor or limit switch likely needs service before it damages the furnace.
Most furnace repairs run $150–$800 in the San Francisco Bay Area, parts and labor — you get a flat-rate quote before any work begins. See our repair cost guide for details.
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