What's actually causing this
Furnaces fail to heat when the filter is clogged (causing overheating and shutdown), the pilot light or ignitor has failed, the thermostat is misconfigured, or the system enters a safety lockout due to a repeated ignition failure.
What you'll need
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Step-by-step fix
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1
Check the thermostat
Set it to HEAT, set the temperature 5 degrees above room temperature, and confirm the fan is set to AUTO. Replace batteries if the display is dim.
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2
Check the filter
A clogged filter causes the furnace to overheat and shut down on a safety limit. Replace the filter if it's gray and dense.
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3
Check the circuit breaker
Find the furnace breaker in the panel. Reset it if tripped. Also check the furnace power switch (looks like a light switch near the unit).
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4
Read the diagnostic LED
Open the furnace access panel and find the small LED on the control board. Count the blinks — most furnaces include a blink code chart on the inside of the panel door.
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5
Check the condensate drain (high-efficiency furnaces)
High-efficiency furnaces produce condensation. If the drain is clogged, the furnace shuts down. Find the PVC drain line and clear it with a wet/dry vac.
- The furnace ignites but shuts off within 30 seconds — flame sensor issue requiring a tech.
- You smell gas — leave the house and call the gas company.
- The furnace is making loud banging or scraping sounds.
- The blink code indicates a pressure switch or inducer motor fault.
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