HVAC, explained

Furnace Won't Ignite: The Ignition Sequence and Where It Fails

A gas furnace lights through a strict sequence of proofs: the thermostat calls, the inducer motor establishes draft, a pressure switch confirms it, the igniter glows or sparks, the gas valve opens, and a flame sensor must prove flame within seconds or the board shuts the gas off and tries again. Where the sequence stalls tells you what failed: silence points to power or controls, clicking without light points to the igniter or gas supply, and a burner that lights then dies moments later is the classic flame sensor failure. Most of the sequence is professional territory because it involves gas and line voltage, and if you ever smell gas, leave the house and call the gas utility from outside before anything else.

A gas furnace is not allowed to simply light. Every attempt runs through a chain of safety proofs, in a fixed order, and the control board refuses to advance until each step demonstrates that the previous one actually happened. This is why a furnace that will not ignite is best understood as a sequence stalling at a specific step rather than a machine that is vaguely broken. In Florida, gas furnaces are the minority, more common in older homes, but they are real, and when one fails the failure usually maps cleanly onto one link in this chain. This guide walks the sequence in order, matches each failure to its symptom, and is blunt about the safety lines that gas equipment draws.

The ignition sequence, step by step

Understanding the sequence is most of the diagnosis, so walk it once in full. It begins when the thermostat calls for heat, closing a low-voltage circuit that tells the furnace control board to start. The board does not light anything yet. First it starts the inducer, a small fan that pulls air through the burner compartment and pushes combustion gases out the flue, clearing any lingering gas and establishing the draft that will carry exhaust safely out of the house. Next, a pressure switch must prove that the draft actually exists; it senses the suction the inducer creates, and only when it closes does the board believe the flue path is clear and moving air. With draft proven, ignition begins: on most modern furnaces a hot surface igniter, a small ceramic element, glows visibly orange over several seconds, while older designs snap a spark repeatedly or hold a standing pilot. Only now does the gas valve open, releasing gas across the burners, where it should light immediately off the waiting igniter. The board then demands proof of flame: a flame sensor rod sitting in the fire must report a tiny electrical signal that can only exist when flame is present, and it must do so within a few seconds. If the proof arrives, the burners stay lit, the heat exchanger warms, and after a deliberate delay the blower starts pushing warm air into the ducts, the delay existing so the blower never distributes cold air from an unwarmed exchanger. If proof of flame never arrives, the board closes the gas valve and starts over. Most boards allow a limited number of attempts, typically around three, before entering lockout, a deliberate refusal to keep trying, because the one thing a furnace must never do is meter gas into a chamber where nothing is lighting it. Every failure in the rest of this article is a stall at one of these gates.

Silence, or clicking that never becomes flame

Start with the quietest failure: the thermostat calls and nothing happens at all, no inducer hum, no clicks, no glow. That pattern sits before the ignition sequence entirely and points to power and controls: the furnace switch, which looks exactly like a light switch and lives on or near the furnace where it gets flipped off by mistake more often than anyone admits; a tripped breaker, reset once and only once; a blower door panel not fully seated against its safety switch; thermostat batteries; or a failed control board. Those first few checks are homeowner-safe, and everything deeper is not. Now the noisier and more famous failure: the furnace clicks but will not light. On a spark-ignition furnace, rhythmic clicking is the igniter doing its job while the gas fails to arrive or fails to light, which points toward the gas valve, gas supply, or a spark too weak to ignite the mixture. Confirm the obvious without touching the equipment: if other gas appliances in the house are also dead, the problem is upstream in the gas supply, not in the furnace. On a hot surface igniter furnace, the equivalent failure is a crack in the ceramic element. These igniters are consumable parts that grow brittle with heating cycles, and a cracked one simply never glows, so the board waits, times out, and retries. After its allotted attempts, the board locks out, and many furnaces blink a diagnostic code on a small LED visible through a window in the panel, worth noting and reporting when you call. Cycling power at the furnace switch clears a lockout and buys one more round of attempts, which is occasionally useful to confirm the symptom, but understand what it is: clearing the lockout does not repair whatever caused three failed ignitions, and repeatedly forcing retry cycles on a furnace that will not light is exactly the behavior the lockout exists to prevent.

Before the flame: the inducer and the pressure switch

Two failures stall the sequence before ignition is ever attempted, and they share a symptom family: the furnace tries, hums or whirs briefly, and gives up without a single click or glow. The first is the inducer itself. Its bearings wear like any small motor, and a failing inducer may screech, rattle, hum without spinning, or run noticeably weaker than it once did. If the inducer never runs, the pressure switch can never close, and the board correctly refuses to go further. The second is the pressure switch side: the inducer runs, audibly, but the switch never proves draft. Sometimes the switch itself has failed, but just as often the switch is telling the truth about a real airflow problem, and this is the case worth respecting, because the pressure switch is the furnace's assurance that combustion gases will leave the house instead of entering it. A flue blocked by debris or a nest, a disconnected or kinked sensing tube, or, on condensing furnaces, condensate backing up where it should drain, all legitimately prevent the proof of draft. Condensing furnaces complicate this in a particular way: they produce water as a byproduct of extracting extra heat from the exhaust, that water must drain continuously, and a clogged condensate trap or drain line can block the pressure switch tubing or trip a drain safety, producing a furnace that starts its sequence and quits before ignition, over and over. The homeowner-safe portion of this territory is observational only: listen for whether the inducer runs, note whether it sounds labored or loud, and look at the outside flue termination for obvious blockage from ground level. Everything beyond observation, opening the burner compartment, testing switches, clearing flue or condensate blockages, is professional work, both because it sits against the combustion side of the machine and because a bypassed or misdiagnosed pressure switch removes the one proof standing between the exhaust and your living space.

Lights, then dies: the flame sensor

One failure pattern is so distinctive it names its own diagnosis: the burners light, run for a few seconds, and shut off, then the furnace tries again, lights again, dies again, and eventually locks out. That is the flame sensor failing to prove a flame that is actually burning. The sensor is a simple metal rod positioned in the flame path, and it works by flame rectification: a burning flame conducts a tiny electrical current from the rod through the flame to ground, and the presence of that current is the board's only evidence that fire exists. The mechanism's weakness is the rod's surface. Combustion gradually deposits an insulating layer of oxide and residue on the sensor, the layer chokes the already tiny signal, and eventually the board can no longer distinguish a healthy flame from no flame at all. Its safety logic then does exactly what it should: gas valve closed, retry, and after repeated failures, lockout. The furnace is not failing to light; it is failing to believe it lit. This failure grows more likely with each season of use, and a furnace that has started dropping its flame occasionally is a furnace headed toward not running at all on some future cold morning. The repair is often humble, cleaning or replacing the rod, but it is best treated as a professional task rather than a homeowner one, for reasons beyond the rod itself. Reaching the sensor means opening the burner compartment of a gas appliance; the rod and its ceramic insulator are fragile and easily ruined by enthusiastic cleaning; and the light-then-die symptom is not exclusive to dirty sensors, since a poor ground connection, a cracked insulator, a marginal gas supply, or a board fault can wear the same disguise. A technician confirms the sensor is actually the culprit by measuring the flame signal directly, which turns a guess into a diagnosis, and the visit doubles as a look at the burners and heat exchanger that the rod sits inside.

If you smell gas, stop reading and leave

Everything else in this article assumes the failure is an absence of fire. A gas smell is the opposite problem and it suspends all troubleshooting instantly. Natural gas is odorized specifically so that a leak announces itself as that unmistakable sulfur, rotten-egg smell, and the correct response is rehearsed and unconditional. Leave the house immediately, taking everyone with you. On the way out, do not operate anything that can make a spark, and the list is longer than intuition suggests: light switches in either direction, appliances, thermostats, garage door openers, and phones, which should stay in your pocket until you are outside and away. Do not start a vehicle inside an attached garage. Do not open windows to air the house out first; the priority is distance, not ventilation. Once you are safely away from the building, call the gas utility's emergency line, or the fire department, and let them clear the house before anyone re-enters. Utilities treat these calls as emergencies at any hour and would far rather respond to a false alarm than the alternative, so the faint-smell-probably-nothing instinct deserves no vote. Separately from emergencies, one boundary deserves plain words: there is no homeowner version of gas-side repair. Burner assemblies, gas valves, gas piping, flue and venting work, and anything involving the heat exchanger, where a crack can introduce combustion gases into household air, are professional work in every case, and in most jurisdictions legally restricted work besides. The stakes are not the furnace; they are fire, explosion, and carbon monoxide, which is colorless, odorless, and produced by exactly the incomplete combustion that a misadjusted or damaged furnace creates. This is also why a home with any gas appliance should have working carbon monoxide alarms regardless of how well the furnace is behaving. The rest of this guide encourages curiosity; this section is the exception, and the entire skill is knowing when to walk away and hand the problem to people equipped for it.

Florida furnaces, and the checks that are actually yours

Gas furnaces are a minority heating system in Florida, where heat pumps and electric strips dominate, but the minority is real: older neighborhoods, homes built with gas service for water heating and cooking, and some inland areas carry plenty of them. Their rhythm here is harder on them than the mileage suggests, because a Florida furnace runs a handful of cold snaps per year and sits idle the rest, and idle seasons are how igniters crack quietly, sensors oxidize undisturbed, and inducer bearings stiffen, all discovered on the same first cold morning. If your heat comes from a heat pump rather than burners, our Florida heater guide covers that system's very different failure map, and if you are unsure which you have, the presence of a flue pipe, burners, and a gas meter feeding the furnace answers it. For furnace owners, the homeowner-safe list is short and worth stating exactly. Confirm the thermostat is calling: heat mode, setpoint above room temperature, fresh batteries. Check the furnace switch, the light-switch lookalike on or beside the cabinet, and confirm the blower door panel is seated so its safety switch engages. Reset a tripped breaker one time, never more. Check the filter, because a suffocating filter can overheat the furnace and trip its limit safeties into short, frustrating heating cycles. Observe and take notes: whether the inducer hums, whether clicks come and how fast, whether anything glows through the sight window, whether flame appears and dies, and what any diagnostic LED blinks. That is the complete list. Everything past it, igniters, sensors, pressure switches, valves, boards, venting, sits on the combustion side of a gas appliance, and the sequence-based observations you just collected are precisely what makes a professional visit short and accurate. A furnace that will not ignite is stalling at one specific gate in a well-understood chain, and telling the technician which gate is a genuinely valuable head start.

Frequently asked

Why does my furnace click over and over but never light?

Repeated clicking is a spark igniter firing while ignition fails to happen, which points to gas not arriving or not lighting: a gas valve problem, a supply problem, or a weak spark. If other gas appliances in the house are also out, suspect the supply rather than the furnace. On furnaces with a hot surface igniter, the parallel failure is a cracked igniter that never glows. Either way, the board will retry a limited number of times and then lock out, and the repair is professional work because it sits on the gas side.

My furnace lights, runs a few seconds, and shuts off. What is that?

That light-then-die pattern is the classic flame sensor failure. The sensor rod proves flame by passing a tiny current through the fire, combustion residue gradually insulates the rod, and eventually the board cannot detect the flame that is genuinely burning, so it closes the gas valve for safety and retries until lockout. Cleaning or replacing the rod usually resolves it, but the same symptom can come from grounding, insulator, or gas supply problems, so it is best confirmed by a technician measuring the actual flame signal.

My furnace locked out after a few tries. Can I just reset it?

Cycling power at the furnace switch clears most lockouts and grants another round of ignition attempts, and doing that once to observe the symptom is reasonable. But the lockout exists because the furnace repeatedly opened its gas valve without establishing flame, and repeatedly clearing it to force more attempts works against the safety logic protecting your home. If a furnace locks out more than once, note any diagnostic light pattern on the control board and make the call rather than continuing to reset.

What should I do if I smell gas near the furnace?

Leave the house immediately with everyone in it. Do not flip switches on or off, do not use phones or anything that could spark until you are outside, and do not stop to open windows. From a safe distance, call the gas utility's emergency line or the fire department and let them clear the home before anyone goes back in. Gas utilities treat these calls as emergencies and prefer false alarms to the alternative. Gas leak repair is never a do-it-yourself task under any circumstances.

Is cleaning the flame sensor a DIY job?

It is commonly presented as one, but it is better treated as professional work. Reaching the sensor means opening the burner compartment of a gas appliance, the rod and its ceramic insulator are fragile, and the light-then-die symptom it produces has several look-alike causes that only a flame signal measurement can separate. A technician can confirm the diagnosis, clean or replace the rod correctly, and inspect the burners and heat exchanger while the compartment is open, which is worthwhile on a furnace that sits idle most of the Florida year.

Do I need to call a pro for this?

Yes. An ignition sequence that fails is a gas appliance refusing to light, and every part of diagnosing it - the valve, the sensor, the igniter, the venting - is licensed contractor work. There is one homeowner step before you call: confirm the thermostat is set to heat and calling. If you smell gas, leave the house first and ring the gas company from outside.

Blume Mechanical Service is a licensed commercial and residential HVAC contractor serving Florida's Tampa Bay region (CAC-1822618). If your furnace is stalling somewhere in its ignition sequence, the observations above plus a diagnostic visit will find the gate it is failing at.

Last updated: 2026-11-05

Sources: General gas furnace control principles (ignition sequences, flame rectification, draft proving) · US Department of Energy — furnaces and home heating systems · Gas utility and fire safety guidance on gas leak response (generalized)

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