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Quick, low-risk steps
- If you smell gas or hear a CO alarm, leave the house immediately and call 911 or your utility emergency line.
- Check that the thermostat is set to 'heat' and the temperature is set above the current room temperature.
- Inspect the air filter if accessible without opening the furnace cabinet; a clogged filter can cause overheating and shutdown.
- Look for a diagnostic light or error code on the furnace and write it down.
- Avoid pressing the reset button more than once; repeated resets can be dangerous.
- Call a licensed HVAC professional if the furnace still won't start.
Immediate Safety Checks
Your safety comes first. Before you touch the furnace or thermostat, use your senses. A gas leak smells like rotten eggs. A carbon monoxide alarm beeping is a life-threatening emergency. Do not ignore these warnings.
If you suspect a gas leak or CO presence, get everyone out of the house. Do not use light switches, phones, or anything that could create a spark. Call for help from outside.
- Evacuate if you smell gas or hear a CO alarm.
- Do not turn on lights or appliances if you suspect a gas leak.
- Call 911 or your gas utility from outside the home.
- Do not re-enter until emergency personnel say it's safe.
Simple Checks You Can Do
If there are no gas or CO dangers, a few basic checks might solve the problem. Start with the thermostat. Make sure it's set to 'heat' and the desired temperature is higher than the room temperature. Replace the batteries if the display is blank.
A dirty air filter can restrict airflow and cause the furnace to overheat and shut down. Check the filter and replace it if it's clogged. Also, ensure all supply and return vents are open and unblocked.
- Confirm thermostat is on 'heat' and set correctly.
- Replace thermostat batteries if needed.
- Check the air filter and replace if dirty.
- Ensure vents are open and not blocked by furniture.
When to Stop and Call a Professional
If the simple checks don't work, it's time to call a licensed HVAC technician. Do not repeatedly press the reset button. Furnaces have safety controls that shut the system down to prevent damage or danger. Bypassing them can cause a fire or carbon monoxide leak.
Write down the furnace brand, model number, and any error code flashing on the control board. This information helps the technician prepare for the visit. Also, note any unusual sounds or smells you noticed before it stopped working.
- Stop troubleshooting if the furnace doesn't start after basic checks.
- Never bypass safety switches or hold down the reset button.
- Record the furnace model and any error codes.
- Describe any strange noises or odors to the technician.
Preparing for the Service Call
When you call for service, ask about the diagnostic fee and what it includes. A reputable company will explain their pricing before they arrive. Ask if they charge extra for after-hours or weekend calls.
During the visit, the technician should perform a full safety inspection, including checking for carbon monoxide leaks and testing combustion. After the repair, ask for a written report of what was done and any parts replaced.
- Ask about diagnostic fees and after-hours charges upfront.
- Ensure the technician checks for CO leaks and combustion safety.
- Request a detailed invoice with parts and labor.
- Ask if the repair is covered under warranty.
Take this with you
Evidence and scope checklist
- Did you evacuate if there was a gas smell or CO alarm?
- Is the thermostat set to 'heat' and calling for heat?
- Did you check the air filter and replace if dirty?
- Are all vents open and unblocked?
- Did you record the furnace model and any error code?
- Did you avoid pressing the reset button more than once?
- Did you ask about diagnostic fees before scheduling?
- Will the technician perform a combustion safety check?
Common questions
What homeowners ask next
Why does my furnace not turn on even though the thermostat is set?
Several issues can prevent a furnace from starting, including a tripped circuit breaker, a dirty air filter causing overheating, a faulty ignitor, or a gas supply problem. A professional can diagnose the exact cause.
Is it safe to keep resetting my furnace?
No. Repeatedly resetting a furnace can be dangerous. The safety switch trips for a reason, often to prevent overheating or a gas leak. If it trips more than once, call a technician.
How much does a furnace repair cost?
Costs vary widely depending on the problem, parts needed, and your location. A diagnostic fee typically applies, which may be credited toward the repair if you proceed. Always get a written estimate before work begins.
Evidence
Sources used for this guide
Research checked July 31, 2026
- Home heating and carbon monoxide safetyU.S. Consumer Product Safety Commission
CO and heating safety; emergencies take priority over troubleshooting.
- Heating and cooling maintenance checklistENERGY STAR
Maintenance context and professional service boundaries.
Optional next step
Need an on-site professional?
Use the guide and worksheet first so you can describe the situation and compare the full scope. A referral is optional; you can also contact a licensed local provider directly.
We have not enabled a provider route for this site. Use the checklist to contact and verify local providers yourself; urgent hazards should go to the appropriate emergency or public service.