Furnace Won't Ignite: Reading the Sequence to Find What Failed

Last updated October 4, 2026 | By Michael Haines, Founder, AC Direct

When a gas furnace won't ignite, the point where its start-up sequence stops tells you why. No igniter glow usually means the pressure switch or the igniter; a glow with no flame points to the gas supply or gas valve; flames that light and die within seconds usually mean a dirty flame sensor. After three failed tries most furnaces lock out and flash an error code.

If the house is cold and the furnace is making noises but not making heat, the temptation is to assume one bad part. That is almost never the shape of the problem. A furnace is a sequence of safety checks that happen to end in flame, and when a check fails the sequence stops exactly there. Watch where it stops and the number of likely suspects shrinks from "everything in the cabinet" to usually two or three parts.

This article is a diagnostic article, not a repair article. We will walk through what should happen when a gas furnace starts, what it means when the sequence stalls at each step, and the handful of checks a homeowner can make without opening the cabinet or touching gas. Everything past that line belongs to a licensed technician, and we will be honest about where that line sits.

AC Direct sells HVAC equipment, including a full line of gas furnaces from Goodman and ACiQ, shipped nationally with no installation markup. We do not repair furnaces and we do not sell replacement ignition parts. If the honest answer to your sequence is "this unit is at end of life," that is where we come in. Everywhere else, your local tech is the right call.

What should happen when a furnace starts?

A modern gas furnace starts in a fixed order. The thermostat calls for heat, the inducer motor runs for roughly fifteen seconds to purge the heat exchanger, the pressure switch closes to prove venting, the hot surface igniter glows for about twenty seconds, the gas valve opens, the burners light within a four to seven second trial window, the flame sensor confirms the flame, the igniter de-energizes, and the blower starts.

That is the whole dance, and every step has a safety interlock. Miss one and the next one does not happen. The point of the fixed ignition-trial window is to prevent unburned gas from accumulating in the burner box after a failed attempt. That is why a furnace does not just "try harder" when something goes wrong. It stops.

Here is the useful part for diagnosis. Most furnace cabinets have a small sight glass on the blower door. Through that window you can see the igniter glow and read the blink code from the control board, both without opening the cabinet and tripping the door switch. That little window is doing more work than most homeowners realize.

What if the inducer runs but the igniter never glows?

If you hear the inducer motor spin up and you feel exhaust at the flue outside, but the igniter never comes to a bright orange glow, the sequence has stopped at the pressure switch. The control board will not energize the igniter until that switch proves venting. If the switch does not close, nothing downstream of it happens.

The pressure switch is a safety device. It is normally open. The inducer motor pulls combustion gases out, which creates a small vacuum in the heat exchanger, which pulls a diaphragm in the switch, which closes the contacts. That closure is the furnace's way of saying "the exhaust has somewhere to go." If any part of that path is wrong, the switch stays open and the furnace freezes mid-sequence.

So what actually keeps that switch from closing? Here is the short list:

• A blocked flue or intake termination. Leaves, snow, ice, a wasp nest, a bird. The outdoor caps are the first place to look.
• A clogged condensate line (high-efficiency models). Condensing furnaces produce acidic water that drains away. Back it up and you can block a pressure port or trip an overflow safety.
• A disconnected or waterlogged pressure switch tube. The small rubber hose running to the switch can crack, pop off, or fill with condensate.
• A weak or failing inducer. The motor runs, but the wheel is not moving the volume of air it used to.
• The switch itself. Switches fail. Diaphragms stiffen with age.

You can safely check the outdoor flue termination for an obvious blockage. You can safely check that the condensate drain is not visibly overflowing. Everything past that, including testing the switch, inspecting the tube or replacing any of these parts, is a technician's job. The pressure switch is a safety device, and bypassing a safety device is not a shortcut, it is the thing those devices exist to prevent. For a deeper look at how this one component thinks, see our breakdown of the furnace pressure switch.

What if the igniter glows but the burners don't light?

If the igniter is clearly glowing bright orange through the sight glass, but the burners never light, the furnace has made it all the way to the gas valve and then stalled. The ignition hardware is doing its job. Gas is not arriving at the burners, or it is not arriving in a form that will burn.

Start with the thing anyone forgets. Is gas getting to the furnace at all? If your gas range lights and your gas water heater runs hot, the house has gas. If no gas appliance in the house works, the problem is upstream of the furnace and belongs to the utility.

Second, the manual shut-off valve on the gas line within a few feet of the furnace. Handle parallel to the pipe is open. Handle crosswise is closed. More than one furnace service call has ended with "someone closed this during the plumbing work last spring and nobody opened it back up." If you smell gas when you look, stop, leave the area, and call the gas utility. That is not a troubleshooting moment.

Beyond those two checks, the likely culprit is the gas valve itself or the signal commanding it. The valve is an electrically controlled solenoid. The control board sends it a low-voltage signal during the trial window, and the solenoid opens the port. If the coil has failed, if the valve has stuck mechanically, or if the control board is not sending the signal, no gas reaches the burners even though the igniter is perfectly hot. A technician will meter voltage at the valve during the ignition attempt to tell which side of that relationship has failed. That diagnosis, and any replacement, belongs to the tech.

What if the flame lights and then goes out?

If the burners light cleanly, run for a few seconds, and then shut off with a soft "whoof" as the gas valve closes, the furnace saw a flame and then decided it did not. This is the signature pattern of a flame sensor problem, and it is one of the most common failures in a gas furnace.

The flame sensor is a small metal rod that sits in the path of the burner flame. It proves the flame electrically. The flame itself conducts a tiny direct current from the sensor back through the burner assembly to the control board, somewhere in the range of one to four microamps on most equipment. If the current the board sees falls below the threshold, the board rules "no flame" and shuts the gas valve, exactly as it should. A furnace that cannot prove its flame is a furnace that cannot be allowed to run.

What changes that tiny current? Usually, a nearly invisible coating of combustion residue on the sensor rod. Soot, oxidation, dust, a film from household aerosols pulled in through the return air. The rod can look clean to the eye and still be electrically insulated. Technicians fix this in two minutes with a strip of fine abrasive. Our walkthrough on spotting a faulty flame sensor covers what the symptom looks like in detail.

A few other things can mimic a dirty sensor:

• Poor furnace ground. The flame rectification circuit needs a clean electrical ground to work. A corroded burner bracket or a loose chassis ground can starve the signal.
• A weak or unstable flame. If gas pressure is low or combustion air is restricted, the flame may not sit on the sensor consistently. We cover the pressure side in what a gas furnace pressure should be when running.
• A cracked heat exchanger. This is the serious one. A compromised exchanger can let combustion gases disrupt the flame, producing visibly floating, rolling or distorted burner flames. Any of those warrants an immediate technician inspection, not a reset.

What does the blinking light mean?

The blinking LED visible through the sight glass is the control board's error code. Every modern integrated furnace board monitors each step of the ignition sequence against a time limit. When a step fails, the board logs the specific fault and, after a set number of ignition retries, puts the furnace into lockout and flashes a code that corresponds to what went wrong.

Most residential gas furnaces allow three ignition attempts before going into lockout. That number is not arbitrary. The point of lockout is to prevent unburned gas from accumulating in the burner box after repeated failed tries. Lockout is the furnace doing its job, not the furnace giving up.

The blink code sequence is manufacturer-specific. A three-flash code might mean "pressure switch did not close" on one board and "flame not detected" on another. The legend is almost always printed on a sticker inside the furnace door, or in the installation manual that came with the unit. Count the flashes, note the pause, count again to be sure.

Lockout can usually be cleared one of two ways. The furnace will typically reset itself after about an hour and attempt the sequence again. Or you can cut power at the dedicated furnace switch or the breaker for roughly twenty seconds, which forces a reset. Here is the thing nobody tells you. If you reset the furnace before you read the code, you have thrown away the single most valuable piece of information you had. Count the flashes first. Write the number down. Then reset.

What can you safely check yourself?

Before calling a technician, there is a short list of checks that are genuinely safe for a homeowner: nothing on this list requires opening the furnace cabinet, touching wiring or dealing with gas. The goal is to rule out the easy things and to give the tech better information when you do call.

Those seven checks resolve a meaningful share of furnace no-ignition calls. If none of them bring the furnace back, you have handed a technician a very short list of remaining suspects. That is the whole point of doing them.

When is it time for a technician, or a new furnace?

Any ignition problem the homeowner checks above did not solve is a technician call. That includes a pressure switch that will not close, a dead igniter, a gas valve that will not open, a flame sensor that keeps failing even after cleaning, a repeating lockout code, any smell of gas, any carbon monoxide alarm, and any visible damage to the burner assembly or heat exchanger. None of those are things a homeowner should take on.

Where it gets harder is deciding whether to repair the furnace again or replace it. Here is the honest test.

Age

Residential gas furnaces have long service lives, but they do not last forever. ENERGY STAR recommends replacing a gas furnace that is more than 15 years old, which is a useful rule of thumb when you are weighing one more repair against a new cabinet. If your unit is past that mark and the quote is substantial, the age by itself is reason to consider replacement.

Repeat failures

One repair in year ten is maintenance. Three service calls in two winters, each for a different part, is a pattern. The Building America Solution Center guidance on pre-retrofit HVAC assessment frames aging equipment as an opportunity to assess the whole system before committing to another round of component swaps. If parts keep failing in sequence, that whole-system look is worth having.

The heat exchanger

If a technician diagnoses a cracked heat exchanger, that is the end of the discussion for most residential furnaces. Industry practice is to replace the heat exchanger (or the furnace) rather than attempt a repair, because combustion gases crossing into the airstream is not a tolerable failure mode. On an older unit, replacing the heat exchanger approaches or exceeds the cost of a new furnace. The visual tells (floating flames, flame rollout, flame distortion) are what a competent tech is checking for whenever a flame-related code appears.

The simple arithmetic

For context on the replacement side, a professionally installed furnace is the equipment plus labor, venting, gas work and permits, and the installation side is usually the larger share. If a single repair quote approaches half of a replacement quote, replacement is usually the better move. You get a fresh warranty, modern efficiency, and the clock resets. Repair makes sense when the unit is young, the fault is a cheap part, and nothing else on it is showing its age. Your installer's quote is what actually governs.

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