Furnace not working: the checks to do before you call

Most no-heat calls start with something you can check in five minutes.

The answer in 2 lines. First make sure there's no gas smell and no sign of carbon monoxide: if there is, get out and call 911 or your gas utility from outside. Otherwise, five no-tool checks often bring a forced-air furnace back: the thermostat (mode, setting, batteries), the furnace power switch, the breaker, the air filter and the blower door. Note any blinking light code. If the furnace still won't run, or shuts down again, call a licensed HVAC technician; renters call the landlord.

The first 3 steps

Make sure there's no gas smell and nobody feels sick (headache, dizziness, nausea). If in doubt, get out and call from outside.
Note what you see: thermostat screen blank or on, furnace silent or running, blower blowing cold air, a blinking light behind the small window on the furnace.
Do the no-tool checks below, once each, in order.

Stop: danger

Smell of gas (rotten eggs): don't flip switches, light anything or start the car. Leave the house, leave the door open, and once outside call 911 or your gas utility's emergency line (propane: your propane supplier).
Headache, dizziness, nausea or confusion, especially in several people at once or easing when you go outside, or a CO alarm sounding: think carbon monoxide. Get everyone outside into fresh air, then call 911 from outside. Don't go back in until responders say it's safe.
Never remove the burner cover, bypass a safety switch, or keep pressing reset on a furnace that keeps locking out: the shutdown is protecting you.
Never heat your home with a gas oven, a grill or a generator indoors: CPSC warns they can produce deadly carbon monoxide.

The no-tool checks

Thermostat: set to HEAT (not COOL or OFF), a few degrees above room temperature, fan on AUTO. Blank screen? Replace the batteries. Check any schedule or vacation mode.
Furnace power switch: usually a light switch on or near the furnace (it looks like a regular wall switch). Make sure it's ON: people often flip it off by mistake.
Breaker: if the furnace breaker has tripped, and only if the panel is dry and your hands are dry, reset it once. If it trips again, stop (see our guide on breakers that keep tripping).
Air filter: a clogged filter can make the furnace overheat and shut itself off. Slide it out: if it's gray and caked, replace it with the same size printed on its edge.
Blower door: most furnaces won't run if the front panel isn't seated firmly; a small safety switch stops them.
Light code: count the blinks of the status light and look them up on the label inside the door or in the manual.

Carrier's guide walks through the same checks: Carrier — Furnace troubleshooting guide.

What it usually means

"Nothing happens at all": thermostat, power switch, breaker, or the blower door safety switch.
"The fan blows but the air is cold": often the burner didn't light (ignition or flame sensor), or the thermostat fan is set to ON instead of AUTO.
"It starts, then shuts off after a few minutes": often a dirty filter (overheating), a dirty flame sensor or a blocked vent. On high-efficiency furnaces, check outside that the white plastic vent pipes aren't blocked by snow or ice.
"Older furnace with a pilot light": if the pilot is out, follow the relighting steps printed on the furnace label exactly. If you smell gas, don't try.
"Boiler, not furnace" (radiators or baseboards, no ducts): see our guide on boiler pressure.

What you can do yourself, and where to stop

The checks above, a new filter, and clearing snow from outside vents.
Keep the area around the furnace clear: no boxes, laundry, paint cans or other flammables.
Stop there: gas valves, burners, igniters, flame sensors, wiring and the control board are for a licensed HVAC technician. Ask for proof of license and insurance before you hire.

What not to do

Pressing the reset button again and again: repeated lockouts point to a real fault.
Running the furnace with no filter "just for tonight".
Closing lots of supply vents to "save heat": it can make the furnace overheat.
Using the oven or a grill to heat the house.

Who pays, and who to call

Homeowners: a licensed HVAC contractor. If the furnace is under warranty, check the manufacturer's label first.
Renters: tell your landlord in writing right away. In most places, landlords must provide working heat; the exact rules and temperatures depend on your state and city (see our guide on who pays for repairs).
New York City example: during heat season (October 1 to May 31), indoor temperature must be at least 68°F between 6 a.m. and 10 p.m. when it's below 55°F outside, and at least 62°F between 10 p.m. and 6 a.m. whatever the outdoor temperature; no heat is reported to 311 (NYC HPD).

Good to know

ENERGY STAR recommends checking the filter every month in heating season, changing it at least every 3 months, and a yearly tune-up by a contractor before winter.
CPSC urges a yearly professional inspection of fuel-burning heating systems and a working carbon monoxide alarm on every level of the home.
Heating is one of the leading causes of home fires. The U.S. Fire Administration's latest published national trends cover 2014 to 2023; figures for more recent years aren't published yet.

On a cold November morning in Columbus, Ohio, Jenna's house is chilly and the vents blow cold air. No gas smell, nobody feels sick. The thermostat is on HEAT, but the switch on the side of the furnace is off: her son flipped it while looking for the basement light. She turns it back on, the furnace starts, and she labels the switch "FURNACE: KEEP ON".

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on November 1, 2026, for the United States. Confidence: high for the safety steps and the basic checks; medium for causes, which only a technician can confirm, and for local heat rules, which vary by state and city. This guide describes common cases, not a diagnosis.