Why won't my furnace ignite?

By Dana Whitlock. Updated October 2026. Repair or replace?

A furnace that stays silent, or clicks and tries without lighting, is either not getting a call for heat, not getting power or gas, or failing a safety check during ignition. Carrier's guide works from the thermostat outward: power, mode, setpoint, breaker, gas valve position. Those five things solve many no-heat calls, and none require opening the furnace.

Likely causes

Thermostat not calling for heat Safe to check

Dead batteries, Cool or Off mode, or a schedule that has not kicked in will keep the furnace idle. Check the display before anything else.

No power to the furnace Safe to check

A tripped breaker or a service switch near the furnace (it looks like a light switch) turned off by mistake will stop everything. Both are visible from the outside.

Gas supply off Safe to check

Carrier tells owners to confirm the gas valve is open. Look at the supply valve on the pipe feeding the furnace. Leave the furnace's internal gas control alone.

Failed igniter or flame sensor Pro job

Hot surface igniters are brittle and crack with age. A dirty flame sensor lets the burners light for a few seconds and then shuts them off. Both live in the burner compartment and need a licensed HVAC technician.

Safety switch lockout Pro job

Pressure, rollout and limit switches stop ignition when venting, draft or temperature is out of range. Because venting problems can involve carbon monoxide, a technician must find the reason rather than just reset the switch.

Safe things to check first

• Replace thermostat batteries and set it to Heat, with the setpoint at least 3 degrees above room temperature.
• Make sure the service switch on or near the furnace is on.
• Reset a tripped furnace breaker once. If it trips again, leave it off.
• Confirm the gas supply valve on the pipe feeding the furnace is open.
• On an older furnace with a standing pilot, relight only per the instructions printed on the unit's label. If it will not stay lit, stop.
• Check the filter. Some furnaces lock out after repeated overheating.

When to stop and call

If those checks do not bring it back, call a licensed HVAC technician. Carrier says a professional should inspect the gas supply or valve if basic steps fail, and that you should never operate switches or try to relight if you smell gas. In that case, leave the house and call the gas utility or 911 from outside. A sounding carbon monoxide alarm means get to fresh air and call 911.

Repair or replace?

Start with its age. Type the serial from the label:

No-ignition problems are usually a single part. Age decides whether a repair is smart. DOE's analysis puts the average gas furnace at about 21.4 years, and inspector charts give 15 to 25 years. Date yours with the age decoder above. On a furnace past 20 years, ask the technician to inspect the heat exchanger while there. A crack makes replacement the only safe choice, and the range above shows what it costs locally.