Dirty Flame Sensor
The flame sensor confirms ignition. If coated with residue, it can't detect flame and shuts down for safety. Cleaning is a common fix.
Clogged Air Filter
Restricted airflow causes overheating. The limit switch shuts down the furnace to protect the heat exchanger. Replace the filter.
Thermocouple Issues (Older Furnaces)
Standing pilot systems use thermocouples to verify the pilot is lit. A failing thermocouple causes safety shutdown. Replacement is inexpensive.
Limit Switch Tripping
The limit switch protects against overheating. If it trips repeatedly, causes include blocked vents, dirty filters, or blower motor problems.
Draft or Venting Problems
Wind or blocked venting can extinguish flames or cause improper combustion. Check that vents are clear and properly installed.
Gas Pressure Issues
Low gas pressure prevents stable combustion. If other gas appliances work fine, the issue is likely within the furnace system.