Furnace short cycling
Starts, runs briefly, shuts off, repeats. Ranked from 1,600 repairs on furnaces with this symptom — and the first thing to rule out costs nothing.
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 before opening the cabinet. A furnace that short cycles on a limit switch is telling you it is overheating — find out why rather than replacing the limit that is protecting it. Heat-exchanger inspection is licensed-tech work.
If you smell gas, or if the furnace short cycles with a burnt or acrid smell: stop, leave the building and call your gas utility from outside.
Narrow it down
My furnace blower runs constantly — what's wrong?
First, check the thermostat: a fan set to ON rather than AUTO is the single most common cause and it is a free fix. If the fan is on AUTO and the blower still never stops, a limit switch stuck open (38%) or a control board failing to drop the fan relay (31%) is the likely cause.
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Also check — 38%38% of matching repairs
A limit stuck open keeps calling the blower to cool a heat exchanger that is already cold.
Try this first: Free first: set the thermostat fan to AUTO rather than ON. That is the whole problem more often than any part on this page.
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Also check — 31%
CTB-1084
Integrated Furnace Control Board
23 in stock · ships today
$128.00 Pro price $108.80 save $19.20 (15%)31% of matching repairsA welded or stuck fan relay on the board leaves the blower energised after the call for heat ends.
The remaining 31% of matching repairs resolved without a replacement part — a filter, a reset or a thermostat setting.
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