Furnace not working: safe checks to try first

Updated October 26, 2026. General information only, not professional advice.

A furnace that quits on a cold night needs quick attention, but some causes are simple and safe to check yourself. Start with safety, then work through the easy checks below. If none of them bring the heat back, it is time for a technician.

Stop here if you smell gas or a CO alarm goes off

Some furnace problems are emergencies. Do not troubleshoot if:

You smell gas (a rotten egg smell). Do not switch lights or appliances on or off, and do not light anything. Get everyone outside, then call your gas utility or 911 from a safe distance.
A carbon monoxide alarm sounds. The CPSC says that if you suspect CO poisoning, get outside to fresh air immediately, and then call 911.
You see smoke, smell burning, or see scorch marks on or near the furnace. Turn the furnace off at the thermostat and the service switch if it is safe, leave the area, and call 911 if there is any sign of fire.

Only go on to the checks below if none of these apply.

Check the thermostat

Many no-heat calls start here.

Make sure the thermostat is set to Heat, not Cool or Off.
Set the temperature a few degrees above the current room temperature.
If the screen is blank, replace the batteries.
If it runs on a schedule, check that a hold or vacation setting is not keeping it low.

Give the furnace a few minutes to respond after any change.

Check power to the furnace

A furnace needs electricity even if it burns gas.

Breaker panel. Look for a tripped breaker labeled furnace, HVAC or air handler. Reset it once. If it trips again, leave it off and call for repair.
Service switch. Most furnaces have a switch that looks like a light switch on or near the unit. Make sure it is on. People sometimes flip it off by mistake.
Furnace door. Many furnaces have a safety switch that keeps them off when the front panel is loose. Make sure the panel is closed and seated.

Check the air filter

A clogged filter can choke airflow, and some furnaces shut down to protect themselves when they overheat. ENERGY STAR suggests inspecting, cleaning or changing the filter once a month.

Turn the system off at the thermostat.
Slide out the filter and hold it up to a light. If you cannot see light through it, replace it.
Match the size printed on the frame, and point the airflow arrow toward the furnace.

Check vents and the outside pipe

Make sure supply vents and return grilles inside are open and not blocked by furniture or rugs.
Many high efficiency furnaces vent through plastic pipes on an outside wall. After snow or ice, check from the ground that these pipes are not buried or blocked. Clear snow by hand only, gently.

Stay warm safely while you wait

Cold homes lead people to take risks. Avoid them.

Never use a gas oven or stovetop to heat your home. Ready.gov warns this can cause carbon monoxide poisoning, fires and burns.
Never run a generator, grill or camp stove indoors or in a garage.
Space heaters: the U.S. Fire Administration says to keep anything that can burn at least 3 feet from heat sources, and to turn space heaters off when you leave the room or go to bed.
Close doors to unused rooms, gather in one room, and dress in layers.
Make sure you have working CO alarms. The CPSC recommends them on every level of the home and outside sleeping areas.
If your home is getting dangerously cold, ask about warming centers or stay with friends or family.

Protect your pipes

A long outage can freeze water pipes. IBHS advises keeping the thermostat at least 55°F to keep pipes from freezing. If the heat is out, open cabinet doors under sinks on outside walls so warmer room air can reach the pipes, and know where your main water shutoff is in case a pipe bursts.

When to call a pro

Call an HVAC technician if the thermostat, power and filter all check out and there is still no heat, if a breaker keeps tripping, if the furnace starts and then shuts off over and over, or if you hear banging, grinding or squealing. Gas valves, burners, igniters and heat exchangers are jobs for a trained technician, not a homeowner.

Before you hire anyone, verify that they hold any license your state or city requires for HVAC or gas work, and ask for proof of insurance. Get a written estimate before work begins. Once the heat is back, consider the yearly pre-season check-up that ENERGY STAR recommends, so a technician can look over the system before the next cold snap.