Furnace won't turn on or won't light?

What usually causes this

These are the common possibilities, not a diagnosis. A page cannot tell which one it is — that takes a technician testing the system.

Igniter worn out

Hot surface igniters weaken every time they heat and eventually crack. The furnace tries to start but never lights.

Dirty flame sensor

The furnace lights, can't confirm the flame, and shuts the gas off within seconds for safety — then often tries again.

Safety limit tripped

A clogged filter or blocked return lets the furnace overheat, and the limit switch shuts the burners off. The switch is doing its job; the airflow is the problem.

Thermostat, power or door switch

Dead thermostat batteries, a tripped breaker, the furnace power switch turned off, or a loose front panel can all stop a start.

Draft inducer or pressure switch

Before lighting, the furnace proves its venting is clear. A failed inducer motor, a blocked vent or a faulty pressure switch stops the sequence.

Gas supply

A closed gas valve or a supply problem. Other gas appliances not working is a clue.

What you can safely check yourself

• Set the thermostat to Heat, a few degrees above room temperature. Replace batteries if it uses them.
• Check the furnace's power switch (it looks like a light switch near the unit) and its breaker.
• Replace the filter if it's dirty.
• Make sure the furnace's front panel is fully in place — a loose panel keeps it off.
• Check that other gas appliances, such as a water heater or stove, are working.

What we check when we get there

1. 1The technician watches the ignition sequence to see exactly where it fails — inducer, pressure switch, igniter, flame proving or limit.
2. 2The heat exchanger and venting are inspected as a safety matter on any gas heating call.
3. 3If airflow tripped a limit, the filter, blower and returns are checked, because replacing the switch alone just removes the protection.

What it is likely to cost

Flame sensors and igniters are common, moderate repairs. Inducer motors, gas valves and control boards cost more. A cracked heat exchanger is quoted individually and often leads to a replacement conversation.

Heating diagnostic

$89

Flame sensor

From $149

Ignitor

From $199

Pressure switch

From $199

Inducer motor

From $549

Gas valve

From $349

Control board

From $449

Prices are starting points for standard work in the Savannah area. Final price depends on the system and site, and is confirmed before work begins. Permits and code-required upgrades may be additional.

Repair or replace?

Igniters and flame sensors are ordinary wear repairs on a furnace of almost any age. A cracked heat exchanger, or repairs piling up on a furnace over about 15–20 years old, is when replacement is worth comparing.

Questions people ask about this

Why does my furnace light and then shut off?

The most common reason is a dirty flame sensor: the burners light, the furnace can't confirm the flame, and it shuts the gas off within seconds. It needs cleaning or replacement by a technician.

My furnace hasn't run since last winter. Should I test it?

Yes. Run the heat for a few minutes on a mild fall day. Finding a problem then is far better than finding it on the first cold night.

There's a burning smell when the heat first comes on.

A short burning-dust smell on the first run of the season is normal. A smell that persists, or smells electrical, is not — switch off and call.

Where to go next