Understanding why furnaces fail helps you prevent breakdowns and recognize early warning signs. Across the USA, these are the most common causes of furnace breakdowns.
1. Dirty or Clogged Air Filters: Restricted airflow causes the system to overheat and shut down. Replace filters every 1–3 months.
2. Ignitor or Flame Sensor Failure: The ignitor or flame sensor is a common point of failure, especially in gas furnaces. A faulty sensor may cause short cycling or failure to ignite.
3. Thermostat Malfunctions: Dead batteries, faulty wiring, or improper settings are frequent culprits.
4. Blower Motor Failure: The blower motor circulates warm air. Over time, bearings wear out or the capacitor fails.
5. Gas Supply Issues: A closed gas valve, gas line problem, or faulty gas valve can prevent heating.
6. Cracked Heat Exchanger: This is both a breakdown cause and a safety hazard, as it can leak carbon monoxide. If suspected, shut off the furnace and call immediately.
For signs that a breakdown has become an emergency, see what is considered a furnace emergency. If you’re unsure whether to call now or wait, use our emergency vs. wait guide. For steps to take when the furnace dies, see what to do when your furnace stops working. And to keep these issues from happening, follow our furnace prevention guide.