When a gas furnace won’t ignite, the most common causes are a dirty flame sensor, a failed hot surface igniter, a closed gas valve, a clogged filter or condensate drain, or a safety switch that has tripped. Modern furnaces follow a fixed startup sequence, so noticing where it stops tells you where to look. Before anything else: if you smell gas, leave the house and call your gas utility or 911 from outside.
Quick answers
- Smell of gas: don’t touch switches, leave the home, call the gas company from outside.
- Check the thermostat is set to HEAT and above room temperature.
- Replace a dirty filter and make sure the furnace switch is ON.
- Flame lights then goes out after a few seconds: usually a dirty flame sensor.
- No orange glow from the igniter: likely a failed igniter.
How a gas furnace starts
Most furnaces built in the last 25 years follow this sequence:
- The thermostat calls for heat.
- The draft inducer motor starts to vent the combustion chamber.
- The pressure switch confirms the venting is working.
- The hot surface igniter glows bright orange (or a spark igniter clicks).
- The gas valve opens and burners light.
- The flame sensor confirms a flame within a few seconds.
- After a short delay, the blower starts to send warm air through the house.
If any step fails, the control board stops and tries again. After several failed attempts, many furnaces go into a lockout for about an hour.
Start with these quick checks
- Thermostat: set to HEAT and at least 3 degrees above room temperature. Replace the batteries if the screen is dim or blank.
- Furnace power switch: looks like a light switch on or near the unit. It should be ON.
- Breaker: check the furnace breaker in your panel.
- Furnace door: the front panel must be fully closed to press the safety switch.
- Air filter: a clogged filter can make the furnace overheat and shut down. Replace it.
- Gas supply: the gas valve handle on the pipe leading to the furnace should be parallel to the pipe. Make sure other gas appliances work.
- Condensate drain (high-efficiency furnaces): if the drain or trap is clogged, a pressure switch may stop ignition.
Where the sequence stops, and why
| What you observe | Likely cause |
|---|---|
| Nothing happens at all | No power, thermostat, door switch, blown control fuse |
| Inducer runs, no igniter glow | Pressure switch issue, blocked vent or drain, failed igniter |
| Igniter glows, no flame | Gas valve closed or faulty, no gas supply |
| Flame lights, then shuts off in 3–10 seconds | Dirty flame sensor, poor ground |
| Burners run, blower never starts | Blower motor, capacitor or limit switch issue |
| Repeats several times then stops | Lockout after failed attempts |
Fix 1: Clean the flame sensor
The flame sensor is a thin metal rod in front of the burners. Over time it gets a coating that weakens its signal, so the control board thinks there’s no flame. Cleaning it is one of the most common DIY furnace fixes. Step-by-step: Furnace Flame Sensor: How to Clean It Safely.
Fix 2: Check the igniter
Watch through the sight glass when the furnace starts. A working hot surface igniter glows bright orange within a few seconds. If it doesn’t glow, it may be cracked or burned out. Igniters are fragile ceramic parts. Replacement is a moderate DIY job for experienced homeowners but is often best left to a technician, who will also confirm the cause.
Fix 3: Clear the vent and condensate drain
High-efficiency furnaces vent through PVC pipes, often out the side of the house. Snow, ice, leaves or nests can block them. Check outside that both pipes are clear. Inside, look for water in the condensate trap and tubing. A blocked drain is common after a long summer of AC use.
Fix 4: Reset after a lockout
Turn the furnace switch off for 30 seconds and back on, or turn the thermostat off and on. This clears a lockout. If the furnace fails again, the underlying problem remains. Don’t reset repeatedly.
Read the diagnostic light
Most control boards have an LED that blinks a code. The code chart is usually printed on the inside of the furnace door. Read it before calling a technician. Guide: Furnace Blinking Light: How to Read Your Furnace’s Code.
When to call a professional
Call a licensed technician if:
- You smell gas or see scorch marks
- The igniter or gas valve needs replacement and you’re not experienced with gas appliances
- The pressure switch keeps failing (this can indicate venting problems)
- The furnace is more than 15 years old and has repeated failures
Frequently asked questions
Why does my furnace click but not turn on?
Clicking usually comes from relays on the control board or a spark igniter. If the burners don’t light, check the gas supply, igniter and pressure switch.
How long should a furnace take to ignite?
Usually 30 to 90 seconds from the thermostat call, including the inducer pre-purge.
Can I relight my furnace’s pilot light?
Only older furnaces have standing pilot lights. Follow the instructions on the furnace label. Most modern furnaces have no pilot to relight.