Furnace won't ignite / no heat
Ranked from 3,800 repairs on furnaces with this symptom. Narrow it below and the ranking recomputes — the more you can tell us about what your furnace is actually doing, the shorter this list gets.
Before you start — Kill the power before you touch anything gas · electrical safety
Switch the furnace off at the service switch AND at the breaker — they are not the same thing, and the blower can start on its own while your hands are in the cabinet. The parts ranked below are safe DIY swaps with the power off. Gas valve and heat-exchanger work is licensed-tech territory in most jurisdictions.
If you smell gas: stop. Don't switch anything on or off, don't use your phone indoors. Leave the building and call your gas utility from outside.
Narrow it down
Most likely parts ranked from 3,800 repairs
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Also check — 38%
IGN-271W
Universal Hot Surface Igniter Kit
67 in stock · ships today
$34.95 Pro price $29.71 save $5.24 (15%)38% of matching repairsThe igniter is the first thing to fail on an 80% furnace and the most common no-heat call we see. If it never glows orange, it has almost certainly failed open — a multimeter continuity check settles it before you spend anything.
Try this first: Check it first: pull the igniter and test for continuity. An open circuit confirms the diagnosis; a good reading sends you down the list.
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Also check — 31%31% of matching repairs
If the burners light and then die within a few seconds, the flame sensor is dirty or has failed. This is the one that most often needs no part at all.
Try this first: Try cleaning it first — light scuff with an emery cloth or fine sandpaper, 5 minutes, free. That alone fixes about half of these.
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Also check — 14%14% of matching repairs
A blocked flue, a cracked hose or a tired inducer leaves the pressure switch open, and the board will never let the gas valve fire.
Try this first: Check the hose for water or cracks before condemning the switch — a waterlogged hose looks exactly like a dead switch and costs nothing to clear.
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Less likely — 9%
CTB-1084
Integrated Furnace Control Board
23 in stock · ships today
$128.00 Pro price $108.80 save $19.20 (15%)9% of matching repairsWhen the igniter, sensor and pressure switch all test good, the board is next. It is also the most expensive guess on this list, so test the cheap parts first.
Try this first: Watch the diagnostic LED through the door window first — the flash code narrows this fast and costs nothing.
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Less likely — 4%4% of matching repairs
Only on older standing-pilot furnaces, where a failed thermocouple drops the pilot and the gas valve closes. If your furnace has a hot surface igniter rather than a pilot light, this is not your part.
The remaining 4% of matching repairs resolved without a replacement part — a filter, a reset or a thermostat setting.
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Ask a tech — freeCommon questions about this symptom
My furnace igniter never glows — what's wrong?
An igniter that never glows has usually failed open: about 74% of these repairs end with a new hot surface igniter. Pull the igniter and check it for continuity with a multimeter — an open circuit confirms it. If the igniter reads good, suspect the control board (12%) or a pressure switch stuck open (9%), either of which stops the board ever calling for ignition.
See the parts for this →My furnace lights then shuts off after a few seconds — what's wrong?
Burners that light and die within about five seconds are the classic dirty or failed flame sensor pattern — 71% of these repairs end there. Clean the rod with an emery cloth first: that is free and it holds about half the time. If cleaning doesn't hold, replace the sensor. Less commonly the control board's flame-sense circuit has drifted (14%) or the gas valve is dropping out (8%).
See the parts for this →My furnace igniter glows but the burners never light — what's wrong?
If the igniter glows and gas never lights, the gas is not arriving: about 52% of these repairs come back to the gas valve failing to open. Check that the gas is actually on and that the valve is getting 24V before condemning it — and note that gas valve replacement is licensed work in most jurisdictions. A pressure switch stuck open (26%) or a board that never energises the valve (15%) produce the same symptom.
See the parts for this →My furnace is blinking an error code — what does it mean?
Count the flashes and read them against your unit's code chart before buying anything — the code is the furnace telling you exactly what it sees, and it beats any ranking we can offer. Across these repairs the control board accounts for 41%, a pressure switch open or intermittent for 27%, and a failed flame sensor for 18%.
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