Last reviewed: August 2026 · By the HVAC Brief Editorial Team · Our sourcing and editorial standards
A modern gas furnace will not ignite unless every safety interlock in its start sequence proves out. Most no-heat calls are the furnace correctly refusing to run, not the furnace failing.
The sequence is: thermostat calls, inducer proves draught through a pressure switch, igniter heats, gas valve opens, flame sensor confirms flame, blower starts. A failure at any step stops ignition, and the control board usually flashes a code identifying which.
Knowing the sequence tells you which failures are homeowner-checkable and which are not. It also explains the most common pattern: a furnace that lights, runs briefly, then shuts down and retries.
The ignition sequence
- Thermostat calls for heat. A circuit closes and the control board wakes.
- Inducer motor starts and establishes draught through the heat exchanger and flue.
- Pressure switch proves draught. If it does not close, the sequence stops here and nothing lights.
- Igniter energises, either a hot surface igniter or a spark.
- Gas valve opens and the burners light.
- Flame sensor confirms flame within a few seconds. If it does not, the gas valve closes.
- Blower starts after a delay so it does not push cold air.
What each failure looks like
| Symptom | Likely stage | Homeowner checkable |
|---|---|---|
| Nothing happens at all | Power, thermostat, door switch | Yes, partly |
| Inducer runs, nothing lights | Pressure switch or blocked flue | Check flue terminal for blockage |
| Lights, runs a few seconds, shuts off, retries | Flame sensor | No |
| Igniter glows but no flame | Gas supply or gas valve | Check gas is on elsewhere |
| Runs then overheats and cuts out | Airflow, filter or ductwork | Yes, filter first |
| Repeated lockout after several tries | Control board has locked out deliberately | No |
That pattern almost always points at flame sensing. The furnace lights, fails to confirm the flame, and closes the gas valve as designed. It is a safety system working correctly on bad information, and it recurs until the sensing issue is addressed. It is not a homeowner repair.
What to check before calling
The filter, because restricted airflow causes overheat cutouts and is the most common owner-fixable cause. The thermostat mode and batteries. The furnace door switch, since many furnaces will not run with the panel off. Whether other gas appliances work, which separates a furnace fault from a gas supply problem. And the outdoor flue terminal, which can block with snow or debris on sidewall-vented equipment.
Beyond that, the diagnostic requires measurement. Anything involving the gas valve, flame sensing or the heat exchanger is a service call.
Frequently asked questions
Why does my furnace light then shut off?
Most often flame sensing. The furnace lights, fails to confirm flame within a few seconds, and closes the gas valve as a safety response, then retries. It is the safety system working correctly on bad information. After several failed attempts the control board locks out deliberately.
Why is my furnace not igniting at all?
If the inducer runs but nothing lights, the sequence has usually stopped at the pressure switch, which proves draught before gas is allowed to flow. A blocked flue terminal, particularly on sidewall-vented equipment in snow, prevents that switch closing. If nothing happens at all, check power, thermostat and the furnace door switch.
Can I fix a furnace that will not ignite?
The owner-checkable items are the filter, thermostat mode and batteries, the furnace door switch, whether other gas appliances work, and whether the outdoor flue terminal is blocked. Anything involving the gas valve, flame sensing or the heat exchanger requires a qualified technician.
What does a furnace lockout mean?
The control board has stopped attempting ignition after repeated failures, deliberately, to avoid releasing unburned gas. Resetting power clears it, but if the underlying fault persists the furnace will lock out again. Repeated resets without diagnosis are not a fix.
Methodology and limitations
This page describes the standard sequence of operation for modern induced-draught gas furnaces. Sequences vary between manufacturers and models, and the manufacturer’s literature is authoritative for any specific unit.
- No failure frequencies are claimed. No federal source measures residential furnace fault rates.
- Gas appliance work carries safety risk and is not owner serviceable beyond the checks listed.
- Diagnostic codes vary by manufacturer. Read the code chart on the unit rather than assuming a meaning.
The HVAC Brief is an independent trade publication. We do not sell equipment, take manufacturer advertising, or accept payment for placement in our research. Corrections welcome.
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