Likely cause: the furnace switch or breaker

Why this happens

Family Handyman says technicians often find the only fix a furnace needs is to be turned on, and that every furnace has a switch like a standard wall switch on or near it. Trane says that switch may be on the side of the furnace, a nearby wall or a ceiling joist. Trane also says a breaker that trips once and works again after a reset may have been caused by a power surge, but a breaker that keeps tripping points to a more serious problem.

How to fix it

1. Make sure the furnace power switch is On.
2. If the furnace breaker is still tripped, check the panel before you touch it. Open only the hinged panel door. CPSC says not to remove the panel cover behind it, because that is a job for a qualified electrician. If the floor near the panel is wet, a breaker is hot or scorched, or you smell burning at the panel, don't touch anything and call a licensed electrician. CDC says never to turn power on or off while standing in water, and Bob Vila lists a breaker that gets hot and trips often, and a breaker with scorch marks, among the signs of a bad breaker. Otherwise, stand to the side of the panel and turn your face away, as Family Handyman advises, then push the breaker to Off and back to On, as Carrier describes. Do this only once. If you already reset it, leave it alone now, and don't reset it again if it trips.
3. Set the thermostat to Off for a moment, as Honeywell advises before you check the furnace door panel.
4. Look at the furnace's front door panel without removing it. Honeywell and Family Handyman say a safety switch behind it keeps the furnace off if the panel isn't fully in place. If it looks loose or crooked, press it firmly into place so it sits flush, without taking it off. If it's off the furnace or won't sit flush, leave it and tell an HVAC technician. Never tape or hold that switch down. Then set the thermostat to Heat, a few degrees above the room temperature, and give the furnace a few minutes to start.
5. If the furnace now clicks without lighting, or the fan blows cold air, use the guides linked below.

When to stop. If the breaker trips again, leave it off and call an HVAC technician. Trane says not to keep resetting it. If you notice a burning plastic or electrical smell once the furnace runs, Carrier says to turn the furnace off and have a qualified HVAC professional inspect it before you use it again. If you smell gas or rotten eggs, leave the house without touching any switch and call 911 or your gas utility from outside. If a carbon monoxide alarm sounds, or anyone feels dizzy, sick or has a headache, get everyone outside to fresh air and call 911. If you see smoke or flames, get out and call 911.

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