Furnace Stopped Working? Safety First, Then Find the Cause
Updated November 2, 2026 · General information, not HVAC or gas-fitting advice
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Gas smell, hissing, or a carbon monoxide alarm sounding? Stop reading and get out. Do not turn anything on or off and do not look for the cause. From outside, call 911 and your gas utility. Go back in only when responders say it is safe.
Gas smell steps ·
CO alarm steps
The short answer: If you smell gas or a CO alarm is sounding, get everyone outside and call 911. If not, check the thermostat (batteries and settings), the breaker and the filter. Do not open burner or gas parts. If those checks do not fix it, call a licensed HVAC technician.
Safety first. Gas, burners, venting and wiring are jobs for a licensed HVAC professional. This page covers only checks a homeowner can do without opening the furnace or touching gas parts.
Why is my furnace not working?
Our free checker starts with a safety screen, then matches your symptom to likely causes and next steps.
Open the furnace checkerGuides
Carbon monoxide and your furnaceWhat to do when the CO alarm sounds, symptoms, alarm placementSmell gas near your furnaceLeave first, then call 911 and your gas providerFurnace not turning onThermostat, power and filter checks, then call a proFurnace blowing cold airWhy the air is cold and what to check safelyFurnace pilot light outDo you have a pilot, safe relight outline, when to stopThermostat blank or not workingBatteries, breaker and loose wiring checksFurnace short cyclingFilter, limit switch, flame sensor, oversizingHow to change a furnace filterSteps, how often and which MERVFurnace reset and blinking light codesWhy codes are maker-specific and what is safe to tryWhen to call an HVAC technicianSame-day signs and pro-only jobsFurnace maintenance checklistMonthly checks and what a fall tune-up coversRepair or replace a furnaceAge, the 25% rule, efficiency and sizingNo heat tonight: stay warm safelySpace heater rules and what never to do
Common questions
My furnace stopped. What should I check first?
If you smell gas or a CO alarm sounds, leave and call 911. Otherwise, check the thermostat batteries and settings, the breaker, and the filter (Erie Institute of Technology, ENERGY STAR). Then call a licensed HVAC technician.
Can a furnace cause carbon monoxide poisoning?
Yes. The NFPA says malfunctioning or improperly vented furnaces can produce carbon monoxide, which you cannot see or smell.
When should I call an HVAC technician the same day?
ServiceMag says if the furnace cycles and blows cold air, if there is soot or scorching near the burners, or if a CO alarm has ever gone off.
How long does a furnace last?
Minnesota Commerce says most furnaces have a useful life of 15 to 20 years, depending on conditions.