If you smell gas, leave the building without switching anything on or off, and call the gas utility or 911 from outside.
How a furnace lights
When the thermostat calls for heat, a small fan clears the combustion chamber, a pressure switch confirms the vent is open, the igniter heats up, the gas valve opens and a flame sensor confirms the burners have lit. If the sensor does not see a flame, the furnace shuts the gas off within seconds for safety.
After several failed tries, many furnaces lock themselves out for a while. A blinking light on the control board often shows an error code that points at which step failed.
Likely causes
- A flame sensor coated with residue, so the furnace lights and then shuts off
- A cracked or worn igniter that no longer gets hot enough
- The gas supply to the furnace is off, or the gas service to the house is interrupted
- A blocked vent, intake or condensate line tripping a pressure switch
- A clogged filter that has made the furnace overheat and trip a limit switch
What you can safely check
- The thermostat is set to heat, set above the room temperature, and has power
- The furnace’s power switch, which often looks like a light switch nearby, is on
- The air filter is clean
- Other gas appliances in the house are working
- Outdoor vent and intake pipes are clear of snow, leaves or nests
When professional help makes sense
Igniters, flame sensors, gas valves and control boards are the technician’s work. Repeatedly switching a furnace off and on to force it to light can hide a problem with venting or combustion, so a furnace that keeps failing to light is worth having looked at rather than coaxed along.