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Why Is My Furnace Not Working? Free Symptom Checker

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

Three quick steps, about 30 seconds. The checker starts with a safety screen, then matches what the furnace is doing to likely causes and safe next steps.

Step 1. Safety screen. Is any of this true right now?
Step 2. What is the furnace doing?
Step 3. When was the air filter last changed?
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.

How it works

This is our own simple matcher, not a diagnosis. Each result comes from the pages listed below. The symptoms are grouped the way those pages group them: thermostat and power problems (Erie Institute of Technology), airflow, flame sensing and limit-switch patterns (ServiceMag), pilot lights (Budget Heating) and filters (ENERGY STAR, DOE, PNNL). Gas and carbon monoxide steps come from the Iowa Utilities Commission, Connecticut PURA, the NFPA, CPSC, CDC and Madison Fire.

Rather have a pro look at it?

Gas, burner, ignition and venting problems are for a licensed HVAC technician. Our guide covers which signs mean call now.

When to call an HVAC technicianFind a licensed HVAC pro near you

Related guides

Sources
  1. Erie Institute of Technology: HVAC Troubleshooting Guide
  2. ServiceMag (trade publication): Furnace Short Cycling
  3. Budget Heating (HVAC contractor): How to Relight the Pilot Light on Your Gas Furnace
  4. ENERGY STAR: Maintenance Checklist
  5. Iowa Utilities Commission: Natural Gas and Propane Gas Safety
  6. NFPA: Get to Know Carbon Monoxide Alarms