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
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%).
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Most likely — 71%71% of matching repairs
"Lights, then dies within seconds" is the textbook dirty-or-failed flame sensor pattern: the burners prove nothing to the board, so it closes the gas valve and locks out.
Try this first: Try cleaning it first — light scuff with an emery cloth, 5 minutes, free. Fixes about half of these outright.
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Also check — 14%
CTB-1084
Integrated Furnace Control Board
23 in stock · ships today
$128.00 Pro price $108.80 save $19.20 (15%)14% of matching repairsA board whose flame-sense circuit has drifted rejects a perfectly good microamp signal. Suspect it only after a clean sensor still locks out.
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Less likely — 8% Licensed tech install
GSV-36J
36J Series Gas Valve, 24V
Low stock — 12 left · ships today
$96.50 Pro price $82.03 save $14.47 (15%)8% of matching repairsA valve dropping out under load starves the burners a second or two after light-off.
The remaining 7% of matching repairs resolved without a replacement part — a filter, a reset or a thermostat setting.
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