A furnace that will not ignite has stopped at one specific point in a fixed sequence, and which point it is tells you almost everything. Does the inducer run? Does the igniter glow? Does the burner light and then die after a few seconds? Those three questions separate most ignition faults from each other.
Why a sequence beats a list of causes
Most guides to this problem hand you a list of parts that could be at fault. That is not how the furnace works and not how it gets diagnosed.
A gas furnace ignites in a fixed order, and each step is a condition the control board insists on before allowing the next. Goodman’s installation manual for its GM9S8/GC9S8 furnaces sets it out: the thermostat closes, the board self-checks, the board verifies the limit switch is closed and the pressure switch circuit is open, it checks the gas valve circuitry, it energises the induced draft blower, it waits for the pressure switch to close proving exhaust is moving, it runs a pre-purge, it energises the igniter, it waits out the igniter warm-up, it opens the gas valve, and the igniter is de-energised as soon as flame is sensed.
A furnace that will not ignite did not fail generally. It reached one of those steps, could not satisfy it, and stopped. Your job as the person standing in the basement is not to fix it. It is to observe where it stopped, because that converts a vague complaint into a specific fault.
What “won’t ignite” actually covers
Three different failures get described with the same words, and they have almost nothing in common.
It never attempts. No inducer, no igniter glow, no sound beyond perhaps a click. The furnace is not getting as far as trying. This is usually a power, thermostat, door-interlock or safety-circuit problem rather than an ignition one, and it is covered in why a furnace that starts after a power cycle has not been fixed.
It attempts and fails. The inducer runs, the igniter may glow, but no flame establishes. This is the classic ignition failure.
It lights, then goes out. Flame establishes and dies within a few seconds, and the cycle repeats. This is a flame-proving failure, and it is a different fault from the one above despite looking similar from across the room.
Where did yours stop?
Work through these by observation only. Nothing here requires tools or contact with the furnace.
Does the induced draft blower run?
Listen for a small fan starting shortly after the thermostat calls. That is the inducer, and its job is to establish exhaust flow before anything is lit.
If it does not run, the sequence stopped before ignition was ever attempted. If it runs continuously with no further progress, the pressure switch has not confirmed flow. The manufacturer’s own troubleshooting chart lists the causes: a blocked, pinched or improperly connected pressure switch hose; a blocked flue or inlet air pipe; a blocked drain system; a weak induced draft blower; or an incorrect switch set point.
Note how many of those are the venting rather than the switch. A pressure switch that will not close is usually reporting that exhaust is not moving — which is the condition it exists to catch.
Does the igniter glow?
On a modern furnace this is a hot surface igniter, and it glows bright orange during the warm-up period before the gas valve opens. Many furnaces let you see it through a small sight glass.
If the igniter never glows and the inducer is running, the fault is on the ignition side — the manufacturer’s chart lists “replace or realign igniter” among the corrective actions. If it glows and no flame follows, the igniter is doing its job and the problem is elsewhere: gas supply, the gas valve, or the burner itself. The chart’s first listed action for failure to establish flame is to “locate and correct gas interruption.”
Does it light and then die after a few seconds?
This is the most diagnostically useful observation on the page, and it points almost exclusively at flame sensing.
Once the burner lights, the control board must prove the flame is there. It does this with a flame sensor — a small rod in the flame path that carries a tiny current when flame is present. The manual notes the igniter is de-energised as soon as flame is sensed. If the board cannot sense flame within a few seconds of opening the gas valve, it closes the valve. That is the correct and intended behaviour: gas released without a confirmed flame is the condition that must never continue.
So a furnace that lights and dies two or three seconds later is usually not failing to burn. It is failing to prove that it is burning.
Does it try three times and then stop entirely?
That is the design. The manufacturer’s troubleshooting chart describes furnace lockout “due to an excessive number of ignition ‘retries’ (3 total).”
Three attempts, then the board latches the fault and stops. Cutting the power clears that latch, which is why a furnace can be coaxed into another few minutes of running — and why doing so repeatedly is overriding a decision the furnace made rather than fixing anything.
The flame sensor: common cause, and not yours to clean
This deserves its own section because the popular advice is wrong in a specific, checkable way.
The mechanism first. Goodman’s manual explains it plainly: “Under some conditions, the fuel or air supply can create a nearly invisible coating on the flame sensor. This coating acts as an insulator causing a drop in the flame sense signal. If the flame sense signal drops too low the furnace will not sense flame and will lock out.”
So a coated sensor produces exactly the symptom above — lights, then dies — while the burner and gas supply are perfectly healthy. It is one of the most common causes of ignition lockout, and the fix is cleaning rather than replacement.
And the manual heads that section “Flame Sensor (Qualified Servicer Only).” Its instruction is that the sensor “should be carefully cleaned by a qualified servicer using steel wool”, and that after cleaning the flame sense signal should measure what the specification sheet requires.
That last clause is the reason, and it is worth stating because a great deal of online advice tells homeowners to do this themselves. Cleaning the sensor is the easy half. The half that matters is measuring the flame sense signal afterwards in microamps and comparing it against the manufacturer’s figure — because a sensor that has been cleaned but still reads low has not been fixed, and a furnace that runs for a week and locks out again has told you nothing you did not already know. The measurement is the diagnosis. Without it you have performed a maintenance action and confirmed nothing.
Reaching the sensor also means being inside the burner compartment of a gas appliance, which is the boundary described below.
Can you light a modern furnace manually?
No, and the reason is structural rather than cautionary.
A furnace with a hot surface igniter has no pilot to light. There is no standing flame, no accessible lighting port, and no manual gas control designed for it. The gas valve is opened electrically by the control board, and only after the board has satisfied the pressure switch, the limit switch and the igniter warm-up. There is no legitimate way to bypass that from outside, and the sequence exists precisely because releasing gas outside it is the hazard.
Older systems with standing pilots are a different design, and even there the correct procedure is the one printed on the appliance rather than a general instruction from a web page.
Where this stops, and why
Observation is yours. Everything from the burner compartment inward is not, and the specific reason is that the failure mode does not announce itself.
The components involved are the gas valve, the burner assembly, the igniter, the flame sensor and the heat exchanger. A mistake among those does not usually produce an obvious failure — it produces a furnace that runs. Incomplete combustion produces carbon monoxide, which has no smell and no colour. A flame sensor cleaned but never measured produces a furnace that works until it does not. There is no feedback loop that tells an untrained person they have got it wrong, which is why the boundary sits where the manufacturer puts it rather than where convenience would.
The safety devices watching all of this — the limit switch, the rollout switch, the pressure switch — and why making one stop tripping is the wrong goal are covered in the safety switches your furnace uses, and why defeating one is dangerous.
One thing overrides this entire page. If a carbon monoxide alarm is sounding, get everyone out of the building and call emergency services before troubleshooting anything.
What a correct diagnosis looks like
For an ignition fault, a correct diagnosis names the step that failed and the measurement that proves it.
If the finding is the flame sensor, ask for the flame sense signal in microamps, before and after cleaning, against the manufacturer’s specification. That number is the difference between a sensor that was cleaned and a sensor that now works.
If the finding is the pressure switch, ask whether the switch itself was tested or whether the venting was inspected — the flue, the inducer, the drain on a high-efficiency unit, and the termination outside. The manufacturer’s chart sends you to the flue for both “loss of flame after establishment” and for a pressure switch that will not close.
If the finding is the igniter, that is straightforward, but ask what the diagnostic code reported, because an igniter is also the part most easily replaced on suspicion.
And in every case, ask what the control board’s fault code was. Modern furnaces report the failed step directly. A diagnosis that never mentions the code is a diagnosis that ignored the furnace’s own account of what happened.
Frequently asked questions
Why is my igniter working but there is no flame? The igniter is doing its job, so the fault is downstream: gas supply, the gas valve, or the burner. The manufacturer’s first listed corrective action for failure to establish flame is to locate and correct a gas interruption.
Can you manually light a furnace if the igniter is not working? No. A furnace with a hot surface igniter has no pilot and no manual lighting port, and the gas valve is opened electrically only after the control board’s safety checks are satisfied. There is no safe way to bypass that.
My furnace lights and then shuts off after a few seconds. What is that? Almost always flame proving. The board opens the gas valve, cannot confirm flame within a few seconds, and closes the valve. A coated flame sensor is the most common cause, and the manufacturer specifies cleaning by a qualified servicer followed by a flame sense signal measurement.
How do I reset my furnace igniter? There is no igniter reset. Cutting power to the furnace clears the control board’s lockout so it will attempt the sequence again — it does not repair what caused the lockout. If a reset is needed repeatedly, the reset is not the answer.
What is the average lifespan of a furnace igniter? Hot surface igniters are a wear item and fail over time, but a manufacturer figure for your specific model belongs to that model’s documentation rather than a general rule. What matters more diagnostically is whether the igniter glows at all, which separates an igniter fault from a gas or flame-sensing one.
Why does my furnace try three times and then stop? That is designed behaviour. The manufacturer’s chart describes lockout after an excessive number of ignition retries, three in total, after which the board latches the fault rather than continuing to release gas into a failed sequence.
Is a furnace that will not ignite dangerous to leave alone? A furnace that has locked itself out has stopped, which is the safe state. The risk comes from repeatedly forcing it to retry. If you smell gas, leave the area immediately, call 911, then contact Xcel Energy, which supplies gas in Boulder.
Talk to us
Boulder HVAC Pros works across Boulder and the surrounding communities. If your furnace is failing to light, or lighting and dropping out, call (720) 764-0399, or see furnace repair in Boulder.
For how this fits into heating a Boulder home generally, see the full guide to winter heating in Boulder.
Sources
- Goodman, Installation Instructions, GM9S8 / GC9S8 gas furnaces (gas heat sequence of operation, one-stage troubleshooting codes, ignition retries, flame sensor guidance) — https://documents.alpinehomeair.com/product/goodmangm9s8gc9s8installationmanual.pdf
- Home Improvement Stack Exchange, “Goodman GMH95 furnace won’t ignite” (asked 2017-09-04; 4,213 views at time of writing) — https://diy.stackexchange.com/questions/122698/goodman-gmh95-furnace-wont-ignite
- National Fire Protection Association, Carbon Monoxide safety — https://www.nfpa.org/education-and-research/home-fire-safety/carbon-monoxide
- City of Boulder, list of Boulder utilities (gas supplier and gas smell instruction) — https://bouldercolorado.gov/list-boulder-utilities
