Furnace Won’t Ignite: Read the Sequence, Then the Flash Code

A modern furnace does not simply fail to light — it stops at a specific point in a fixed start-up sequence, and it flashes a code telling you where. Reading that code, and understanding the seven steps behind it, is the difference between calling a contractor with “my furnace won’t start” and calling with “it’s locking out with three flashes, the inducer runs but the igniter never glows.” Contractor cost guides put the most common fix — a hot surface igniter — at $150–$450 installed.

Read this first

One more thing belongs at the top, because a furnace that will not light is a combustion appliance behaving unexpectedly. Carbon monoxide is odorless, colorless and toxic, and because it cannot be seen, tasted or smelled, the EPA states it can kill you before you are aware it is in your home. The symptom pattern worth memorising, in the EPA’s own words, is that CO can cause flu-like symptoms that clear up after leaving home — headaches, dizziness, nausea, confusion, impaired vision and coordination that get better when you go out and come back when you return. That is how household CO poisoning is actually recognised. If that pattern fits your household, treat it as an emergency, get everyone out, and call from outside.

If a contractor has told you the heat exchanger is cracked, or you suspect it is, the furnace gets shut off at the gas shutoff and at the electrical disconnect and does not run, not even for one cold night — until it is repaired or replaced. There is no safe way to run it in the meantime. That decision is covered in full in when to replace your furnace.

The ignition sequence, step by step

Nearly every gas furnace built since the early 1990s runs the same start-up sequence, supervised by a control board. Each step has to prove itself before the next one is allowed to happen. That is not fussiness — it is what stops a furnace from filling a cabinet with unburned gas.

1. Call for heat. The thermostat closes a low-voltage circuit and tells the board the house wants heat. If nothing at all happens — no hum, no fan, no light — the problem may be upstream of the furnace entirely; see thermostat not working.
2. The inducer motor runs. A small fan at the top of the furnace spins up and pulls air through the burner compartment and out the flue, clearing any residual gas before ignition. You will hear it as a rising whir, distinctly different from the big indoor blower.
3. The pressure switch proves draft. The inducer creates a slight vacuum, and a switch connected by a small rubber hose senses it and closes. This is the furnace confirming that exhaust actually has somewhere to go. If it cannot prove draft, the sequence stops here and nothing lights.
4. The igniter energises. On most modern furnaces this is a hot surface igniter — a small ceramic element that glows bright orange for 20 to 40 seconds. Some furnaces use an intermittent spark instead, which you hear as a rapid tick-tick-tick.
5. The gas valve opens. Only now does gas reach the burners, and it lights off the glowing igniter or the spark.
6. The flame sensor proves flame. A thin metal rod sits in the flame and carries a tiny electrical current — microamps — through the ionised gas. That current is the furnace’s only proof that the burners actually lit. No current, no proof, and the board shuts the gas valve within a couple of seconds.
7. The blower starts on a delay. Thirty to ninety seconds later, once the heat exchanger has warmed, the indoor blower starts and you finally feel warm air. The delay is deliberate; it is not a fault.

If the furnace tries this sequence several times and fails, it goes into lockout — it stops trying and waits, usually an hour, before attempting again. A hard lockout means it has given up until power is cycled or the fault is fixed. Lockout is the furnace protecting you, not breaking.

Read the flash code first

Before touching anything, look at the furnace. On the front panel there is usually a small sight glass — a plastic window an inch or two across. Behind it, on the control board, is an LED that flashes a pattern: three short flashes, then a pause, then three again. Some boards use a two-digit display or two lights of different colours.

The legend is printed on the inside of your furnace’s own door. Open the outer panel — the one that lifts or slides off without tools, and there will be a label or diagram listing what each pattern means for that specific model. This is the single most useful thing in the whole machine and few people know it is there.

Write down what you see — how many flashes, whether it repeats, whether the pattern changes when the thermostat calls for heat. That one sentence, given over the phone, often shortens a diagnostic visit substantially.

Where the sequence usually stops

Working in sequence order, because that is how a technician works:

Nothing happens at all

No inducer, no lights, no sound. Check that the furnace’s own service switch — a light-switch-looking toggle on or near the cabinet — has not been flipped, and that the breaker has not tripped. Reset a tripped breaker once. If it trips again, stop: the Electrical Safety Foundation International is explicit that a breaker that repeatedly trips indicates a defect that may cause fire or shock. That is a call, not a reset. Also check that the furnace door is properly closed — there is a door interlock switch that kills power when the panel is off, which is a safety device and must never be taped down or defeated.

The inducer does not run

The board is calling but the little fan does not spin, or it groans and stalls. That points at the draft inducer motor or assembly, or at the board itself. Contractor cost guides put a draft inducer replacement at $400–$1,200 installed, wide because some furnaces take a bare motor and others need the whole assembly.

The inducer runs but the sequence stops there

This is the pressure switch failing to close, and here is the part that matters: the switch is usually right. It is telling you the furnace cannot prove draft. The common reasons are a blocked or obstructed flue, a cracked or disconnected hose to the switch, and — on a high-efficiency condensing furnace — a blocked or frozen condensate drain. On a condensing furnace the condensate path and the combustion safety chain are physically linked: water backs up, the switch cannot make, and the furnace locks out. A homeowner sees “dead furnace” and the actual fault is a plugged plastic tube.

Contractor cost guides put a pressure switch replacement at $150–$450 installed, and a proper job includes checking the vent and the condensate trap for the blockage that caused it.

The igniter never glows

You hear the inducer, you wait, and no orange glow appears through the sight glass. The usual culprit is the hot surface igniter, which is a consumable — a brittle ceramic element that cracks with age and thermal cycling. It is the single most commonly replaced furnace part and it is a legitimate repair on a furnace of any age. Contractor cost guides put it at $150–$450 installed, with the part alone roughly $20–$90 at distributor pricing. It is also the cheapest thing on the fix list and very often the entire problem.

The igniter glows but nothing lights

The element goes bright, the sequence times out, and no flame appears. Either gas is not arriving or the valve is not opening. Check whether the manual gas shutoff at the furnace is open — a handle in line with the pipe is open, a handle crosswise is closed. That is a genuine homeowner check and occasionally the whole answer after other work has been done in the house. Beyond that, a gas valve is not a homeowner job under any circumstances: it is open gas piping, and a leak test of every disturbed joint is part of the job, not an extra. Contractor cost guides put a gas valve replacement at $400–$900 installed.

It lights, then dies after a few seconds

Flame appears, holds for two to five seconds, then vanishes, and the furnace tries again, and again, and eventually locks out. That is the classic flame sensor signature. The rod gets coated with a fine oxide film, the microamp signal drops below threshold, and the board concludes there is no flame even though there was one.

It lights and runs, then shuts down early — over and over

The burners light, the blower comes on, and a few minutes later everything stops and restarts. That is usually a high-limit switch tripping because the heat exchanger is running too hot, and the usual reason is restricted airflow: a blocked filter, closed or blocked returns, a failing blower, or collapsed ductwork. The limit switch is a symptom, not a fault. Replacing it without finding the restriction removes the protection and leaves the cause in place — the heating equivalent of fitting a bigger breaker. Contractor cost guides put a limit switch replacement at $150–$450, and a proper job includes finding the restriction. Repeated overheating is also one of the ways a heat exchanger cracks, which is why this symptom is worth taking seriously rather than living with. If your complaint is cool air rather than short cycling, furnace blowing cold air takes that symptom apart.

A note on “thermocouples”

If a quote names a thermocouple, check the age of your furnace. A thermocouple is a standing-pilot part — it holds the gas valve open while a small pilot flame burns continuously. Furnaces with electronic ignition, which is essentially everything sold since the early 1990s, have no thermocouple at all; they have a hot surface igniter and a flame sensor. A thermocouple named on a 2010 furnace is a description of the wrong part. Contractor cost guides put a genuine thermocouple replacement, on a genuine standing-pilot furnace, at $100–$300. And a furnace old enough to have a pilot light is old enough that its age belongs in the conversation. We will not tell you how to relight a pilot — that instruction belongs with the specific appliance’s own label and its manufacturer, not with a general guide.

The short list of what you can safely do

1. Confirm the thermostat is calling for heat — set to Heat, setpoint above room temperature, fresh batteries if it takes them.
2. Confirm the furnace service switch is on and the door panel is properly seated.
3. Reset a tripped breaker once. Not twice.
4. Confirm the manual gas shutoff at the furnace is open, and that other gas appliances in the house are working — if the water heater and range are also dead, the problem is the supply, not the furnace.
5. Change the filter if it is dirty, especially if the symptom is short cycling. A restricted filter makes a furnace overheat. A standard pleated filter runs under $30 and takes five minutes to change.
6. Read and write down the flash code.

That is the whole list, and it is short on purpose.

Why everything else is a licensed pro job

Inside the cabinet, three things are true at once. There is line-voltage wiring next to a low-voltage control board. There is a gas train — valve, manifold and burners — where a disturbed joint has to be leak-tested. And every safety device in there exists specifically to keep combustion products out of the air your family breathes. Anyone can find a video showing how to jumper past one of them. What the video will not show is the failure mode six weeks later.

Never bypass a pressure switch. Never jumper a limit switch. Never tape or defeat the door interlock. Never run the furnace with the front panel off. And do not open the sealed combustion chamber or attempt burner cleaning; that is a gas-train disassembly.

What each repair costs

Every figure below is a contractor-reported market estimate. No government body or public index publishes installed furnace repair prices — none exists, so these are market ranges to argue from, not posted prices. A diagnostic or service call itself runs $70–$200 in business hours and $150–$500 or more after hours, on a weekend or on a holiday, and is frequently credited against the repair if you go ahead.

Two things to know about that last row. The control board is the part most likely to be no longer available on an older furnace, and unavailability rather than price is what usually forces a replacement decision. It is also the most commonly misdiagnosed item on the list — a board gets condemned when the real fault was a weak flame signal from a dirty sensor. Ask what was measured before you authorise one.

When repairs start stacking up, the comparison to make is against a whole new furnace. Contractor cost guides put an 80% AFUE replacement at $3,500–$6,500 and a 90%-plus condensing furnace at $5,000–$9,000; the full breakdown, including the venting costs that separate those two jobs, is on furnace replacement cost.

When it is an emergency

• Gas or rotten-egg smell — leave now, call 911 and the gas utility from outside. Touch nothing indoors.
• CO alarm sounding — everyone out to fresh air, account for everyone, call for help from outside, stay out until responders say it is safe.
• Flu-like symptoms that clear up when you leave the house — treat as CO until proven otherwise.
• Soot, scorching, or a rumbling or roaring sound at the burners — shut the furnace off and call.
• A breaker that trips again after one reset — leave it off and call.
• No heat at all in freezing weather is an urgency question rather than a safety one, but if there are infants, elderly or medically vulnerable people in the house, treat it as urgent and arrange somewhere warm while you sort the repair.

What to have ready when you call

Five things, and having them ready genuinely changes the visit:

1. The model and serial number, photographed off the rating plate inside the door.
2. The flash code, as a count of flashes.
3. Which step it reaches — inducer only, igniter glows, lights and dies, runs then stops.
4. When it started, and whether anything else changed — a filter, a power cut, work done in the house.
5. The age of the furnace and whether you are the original owner, because warranty coverage on a furnace bought with the house is frequently not what people assume.

FAQ