It’s the second week of November, which means the phones here do a predictable thing every morning: somebody’s furnace clicked, hummed, maybe even glowed orange for a few seconds — and then nothing. No flame, no heat, and a house that’s getting colder while they Google what that clicking was.
Here’s the thing most homeowners don’t know. A gas furnace doesn’t just “turn on.” It runs through a strict sequence of checks, in order, every single time your thermostat calls for heat. Each step has to prove itself before the next one is allowed to happen. And when your furnace won’t light, the point where the sequence stops tells you, almost exactly, which part failed.
We sell the parts that fail. Igniters, flame sensors, pressure switches, inducer motors. Most of them are small, and compared to a service call, most of them are cheap. So this post walks through the whole ignition sequence step by step, what you should hear and see at each stage, and which part to suspect when things stall. Even if you never pick up a screwdriver, knowing where the sequence died makes you a much smarter customer when the tech shows up.
One rule before anything else: if you smell gas, stop reading. Leave the house and call your gas utility from outside. Nothing below applies until that’s resolved.
Table of Contents
- The ignition sequence, step by step
- Where it stalls: matching the symptom to the part
- The flame sensor: the two-dollar-looking rod behind half of these calls
- Hot surface igniters: handle like a lightbulb, because it basically is one
- Pressure switches, and why the problem usually isn’t the switch
- Lockout mode and blinking lights
- What’s safe to DIY and when to call a pro
- Getting the right replacement part
The Ignition Sequence, Step by Step
Stand next to your furnace and turn the thermostat up a few degrees. On a modern furnace with a hot surface igniter — which is most gas furnaces installed in the last couple of decades — here’s the order of operations.
1. The thermostat calls for heat
Your thermostat closes a low-voltage circuit, and the furnace’s control board wakes up. You won’t hear much at this stage. The board runs a quick self-check, making sure safety switches like the high limit and rollout switches are where they should be before anything moves.
2. The inducer motor starts
The first sound you hear is a small motor spinning up: a steady hum or whir from a fan about the size of your fist. That’s the draft inducer. Its job is to pull fresh air through the burner compartment and push exhaust out the flue before any gas flows. Think of it as the furnace clearing its throat.
3. The pressure switch proves the draft
The inducer creates a small vacuum, and a diaphragm switch called the pressure switch feels that vacuum and closes. This is the furnace proving to itself that exhaust actually has somewhere to go. If the switch doesn’t close, the sequence stops right here. The inducer will just run and run, and nothing else happens.
4. The igniter glows
With the draft proven, the board sends power to the hot surface igniter. Through the sight glass you’ll see it glow bright orange, like a tiny toaster element. The warm-up takes somewhere in the neighborhood of fifteen seconds to a minute depending on the furnace. Older furnaces may use a spark igniter instead (a rapid click-click-click), and furnaces older still may have a standing pilot that’s always lit.
5. The gas valve opens and the burners light
Now, and only now, does gas flow. It hits the glowing igniter, the first burner lights with a soft whump, and the flame carries across the remaining burners left to right in about a second. This is the moment everything above was building toward.
6. The flame sensor proves the flame
A thin metal rod sits in the path of the last burner’s flame. It’s called the flame sensor, and within a few seconds of ignition it has to confirm to the board that fire actually exists. If it can’t, the board slams the gas valve shut. That’s not a malfunction — that’s the furnace refusing to pour unburned gas into your house. Most boards will retry the whole sequence a few times before giving up and locking out.
7. The blower comes on
The burners run alone for roughly half a minute to a minute, warming the heat exchanger so the first air out of your vents isn’t cold. Then the main blower kicks on and you finally feel heat. From thermostat click to warm air is typically a couple of minutes, and most of that is deliberate, built-in proving time.
That’s the happy path. Now the useful part.
Where It Stalls: Matching the Symptom to the Part
When a furnace won’t heat, it almost never fails randomly. It fails at a specific step, and the symptom tells you which one.
Nothing happens at all. No hum, no click, nothing. Check the obvious first: furnace power switch (it looks like a light switch, usually on or near the furnace, and gets bumped off more often than anyone admits), the breaker, the thermostat batteries, and the blower door — there’s a safety switch under it, and a door that’s not fully seated kills the whole furnace. If all that checks out, you’re into control board or thermostat wiring territory. We covered thermostat wiring in its own post: Thermostat Wiring Explained.
The inducer runs and runs, but nothing else happens. The pressure switch never closed. Sometimes the switch itself is bad, but more often it’s just the messenger. More on that below.
The inducer runs, you hear clicks or see nothing glow, and no flame. If your furnace has a hot surface igniter and it never glows, the igniter is the prime suspect. They crack. It’s what they do. If it’s a spark unit that clicks away with no light-off, think igniter gap, a fouled pilot assembly, or the gas valve not opening.
The burners light, run for a few seconds, then shut off. Then it tries again. And again. This is the classic one, and it’s the flame sensor’s signature move. The flame is real, but the sensor can’t see it, so the board kills the gas. Three tries, then lockout. If this is your symptom, the odds are good you’re one cleaning away from heat.
The furnace lights fine but shuts down mid-cycle, over and over. That’s short cycling rather than an ignition failure, and it has its own list of causes — dirty filter and tripping limit switch at the top. We wrote that one up separately: Furnace Turns On, Then Shuts Off?
The Flame Sensor: The Humble Rod Behind Half of These Calls
If we had to pick one part responsible for more November no-heat calls than any other, it’s this one. Not because flame sensors break, exactly — they rarely fail outright. They get dirty.
The sensor works by passing a tiny electrical current through the flame itself. Fire conducts electricity, just barely, and the board reads that trickle of current as proof of flame. The signal is so small that a light coating of oxidation or combustion residue on the rod is enough to choke it off. The furnace is burning away happily, the sensor swears there’s no flame, and the board believes the sensor.
The fix is cleaning, and it’s about a ten-minute job: kill power at the switch, close the gas valve, remove the single screw holding the sensor (it’s the thin rod with one wire attached, sitting in the path of the burner farthest from the igniter), and polish the rod gently with a fine abrasive pad or emery cloth. Wipe it clean, reinstall, restore gas and power. Don’t touch the rod with bare fingers after cleaning — skin oils undo your work.
If cleaning doesn’t fix it, or you’re cleaning it every few weeks, replace it. Flame sensors are one of the cheapest parts on the furnace, and they’re a smart thing to keep on the shelf if you’re the type who keeps spare furnace filters around. A surprising number of homeowners have paid an after-hours service rate for what turned out to be thirty seconds with an emery cloth.
Hot Surface Igniters: Handle Like a Lightbulb
The hot surface igniter is a brittle ceramic element that heats to glowing in seconds, thousands of times over its life. Like a lightbulb filament, it wears out, and like a lightbulb, the failure is usually sudden. One cold morning it just doesn’t glow.
Sometimes you can see the problem: a visible crack, or a white, blistered spot on the element. Sometimes you can’t, and the real test is checking resistance across the igniter’s plug with a multimeter. A reading of infinite resistance means the element is broken inside, even if it looks fine. Compare any reading you do get against the spec for your specific igniter, because expected values vary by model. If you don’t own a meter, this is exactly the kind of job that justifies one; we carry UEi test instruments, the same meters a lot of the techs who buy from us keep in the truck.
Two rules for replacing one. First, never touch the new element with your fingers — oils create hot spots that shorten its life. Handle it by the base only. Second, match the part. Igniters come in different shapes, mounting brackets, and voltages, and “it looks close” is how people end up with a furnace that still won’t light. Pull the old part, note your furnace’s full model number, and match both.
A furnace that eats igniters — more than one in a couple of years — is telling you something else is wrong: voltage problems, flame rollout, or a board holding the igniter on too long. That’s a diagnosis call, not a parts call.
Pressure Switches: The Messenger, Not the Problem
When the inducer runs forever and the sequence never advances, it’s tempting to throw a pressure switch at it. Techs will tell you the switch is innocent more often than it’s guilty. The switch didn’t close because it never felt the draft it was waiting for. The interesting question is why.
The usual suspects: a blocked flue or intake pipe (snow, ice, leaves, and the occasional unlucky bird; we wrote a whole post on furnace venting and the snowdrift problem), a kinked or cracked rubber hose running to the switch, a weak inducer motor, or — on high-efficiency furnaces — water. Condensing furnaces make water constantly, and a clogged condensate trap can back water up into the pressure switch tubing. If your 90-plus furnace stalls at this step, check the condensate trap and drain before you condemn anything electrical.
A genuinely stuck or failed switch does happen, and when it does, the same matching rule applies: pressure switches are calibrated to a specific draft, so replace like for like by part number, not by whatever fits the bracket.
Lockout Mode and the Blinking Light
After a few failed ignition attempts, most furnaces lock themselves out for a while (commonly an hour or more) or until power is cycled. This surprises people. The furnace isn’t dead; it’s sulking, on purpose, because something upstream failed repeatedly and it refuses to keep trying.
Before the lockout clears the evidence, look through the sight glass at the control board. There’s almost always a small LED blinking a code, and a legend for the codes printed on the blower door or a label inside the cabinet. Write the code down, counting the flashes, before you cycle power. Whether you’re fixing it yourself or calling someone, walking in with “it flashed three times, which the panel says is a pressure switch fault” turns a fishing expedition into a repair.
To reset, flip the furnace power switch off, wait thirty seconds, and flip it back on. If it lights and runs, fine — but a furnace that needed a reset will usually need another one soon. Find the cause.
What’s Safe to DIY, and When to Call a Pro
Reasonable homeowner territory: checking power, filter, thermostat, and the blower door switch; reading fault codes; cleaning a flame sensor; clearing a blocked intake or flue termination you can reach from the ground; flushing a condensate trap; and replacing a flame sensor or hot surface igniter with power and gas off, if you’re comfortable with basic hand tools.
Pro territory, no argument from us: anything involving the gas valve or gas piping, burner or heat exchanger work, repeated lockouts you can’t explain, any suspicion of a cracked heat exchanger, and combustion testing — which is how a good tech confirms the furnace is burning safely, not just burning. Our post on carbon monoxide and your furnace explains why that distinction matters, and why every home with gas heat needs working CO detectors regardless.
And if the diagnosis comes back as something big on a furnace that’s old enough to vote — a board, an inducer, and a weekend of downtime — it’s worth running the repair-or-replace math before sinking money in. If replacement wins, we carry Goodman, Daikin, and Solace furnaces at distributor-direct pricing, with a price match guarantee and 0% APR financing through Affirm.
Getting the Right Replacement Part
Whatever the sequence pointed you to, the part number does the heavy lifting. Before you order anything, grab three things: your furnace’s full model and serial number (on a label inside the cabinet, usually near the burners or on the blower housing), the number printed on the failed part itself, and a photo of the part installed, including how it mounts and plugs in.
With those in hand, browse our HVAC parts and supplies: igniters, flame sensors, pressure switches, inducer assemblies, control boards, and the small stuff like hoses and gaskets that’s worth replacing while you’re in there. Orders over $99 ship free, which most parts orders with a tool or two tossed in will clear. And if you’re staring at a parts diagram unsure which igniter is yours, call us at (877) 843-4822 or email Info@BuyComfortDirect.com. Matching parts to model numbers is a thing we do all day, and we’d rather spend five minutes on the phone than process a return.
The first real cold snap always finds the weak igniter and the dirty flame sensor. Now you know where in the sequence to look when it finds yours.