The short answer
If you smell gas or a carbon monoxide alarm sounds, leave first and call 911 or your gas utility from outside. Otherwise: check the thermostat is on Heat above room temperature, replace a dirty filter, make sure the furnace switch and breaker are on and the front panel is closed tight, then read the blinking error light.
Safety first
Gas furnaces burn fuel inside your home. If you smell gas – often like rotten eggs – do not switch lights or appliances on or off, get everyone out, and call your gas utility or 911 from outside. If a carbon monoxide alarm sounds, or people feel headachy, dizzy or sick and better outside, leave and call 911. A cracked heat exchanger can leak carbon monoxide; it is a technician’s job to inspect.
The thermostat
Set it to Heat and a few degrees above room temperature. Replace weak batteries. If you have a heat pump with a gas furnace backup, or a smart thermostat, check it is not in a schedule or “away” mode that overrides your setting.
The filter
A clogged filter makes a furnace overheat. A high-temperature safety switch then shuts the burner off – you may notice it lights, runs a few minutes and stops. Replace the filter and try again.
The switch, breaker and door panel
Furnaces have a power switch nearby that looks like a light switch; make sure it is on. Check the breaker. Most furnaces also have a switch that keeps them off when the front panel is loose – push the panel firmly into place.
Read the blinking light
Most modern furnaces have a small window with a status light on the control board. Count the blinks and match the pattern to the chart printed inside the panel door. It may point to a pressure switch, flame sensor, igniter or limit switch. Tell the technician the code when you call – it speeds up the visit.
Vent pipes and the condensate drain
High-efficiency condensing furnaces vent through two plastic pipes, often out a side wall or the roof. Snow, ice, leaves or a nest blocking them will shut the furnace down. Clear them from outside if you can do so safely. These furnaces also make water: a clogged drain or a failed condensate pump can stop them. Look for water around the base.
Gas supply and pilot light
If other gas appliances also fail, the problem may be the gas supply – call your utility. Older furnaces with a standing pilot light may need relighting by following the instructions on the unit. If the pilot will not stay lit, call a technician. Never try to bypass a safety switch.
Furnace runs but blows cold air
Check the fan setting is Auto – on “On” the blower runs between heating cycles. If the burner lights and goes out, a dirty flame sensor or an overheating safety switch is likely. If you have a heat pump, it may be in a defrost cycle, which is normal for a few minutes.
A word on resetting
Turning the power off for a minute and back on can clear a temporary lockout. Do it once. A furnace that locks out again is telling you something is wrong; repeated resets can make it worse. Call a company and give it the error code.
While you wait in freezing weather
- Keep faucets on outside walls dripping and open cabinet doors under sinks to help prevent frozen pipes.
- Use only space heaters made for indoor use, with tip-over shutoff, plugged straight into a wall outlet and well away from anything that burns.
- Never heat a home with an oven, a grill, or a generator running indoors or in a garage.
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Questions
Why does my furnace turn on and then shut off?
Common causes are a dirty flame sensor, a clogged filter that makes the furnace overheat, a blocked vent pipe or condensate drain, or a failing igniter. The blinking light on the control board usually narrows it down.
Why is my furnace blowing cold air?
Check the thermostat fan is on Auto, not On. If the burner is not staying lit, the furnace blows unheated air until its safety controls shut it down. A clogged filter or dirty flame sensor are frequent causes.
Can I reset my furnace?
Turning the power switch off for a minute and back on can clear a temporary lockout. Do it once. If it locks out again, call a technician with the error code.
Is a burning smell from my furnace normal?
A brief dusty smell the first time it runs each season is common. A smell that lasts, any smoke, or a gas smell is not – turn it off and, for gas or smoke, leave and call from outside.
Related guides
Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary