When your furnace suddenly stops working in the middle of a cold snap, it’s easy to panic. But before you reach for the phone, know this: many furnace issues are common and can be solved with a little know-how and a flashlight.
I’m Mark Callahan, HVAC veteran and straight-shooter. I’ve seen homeowners spend hundreds of dollars on service calls that could’ve been avoided with a flipped switch or a filter change. In this guide, I’ll walk you through the most common furnace problems, how to troubleshoot them safely, and when it’s time to call in a professional.
Start Here: Furnace Safety First
Before you pop open the access panel or grab a wrench, make sure you follow basic safety procedures:
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Turn off power to the furnace at the breaker
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Shut off the gas supply (if applicable)
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Never open sealed compartments without proper knowledge
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If you smell gas or suspect a carbon monoxide leak, leave the house and call emergency services immediately
Also, make sure you’ve got a working carbon monoxide detector near your furnace. The National Fire Protection Association (NFPA) provides guidance on correct detector placement.
1. Furnace Won’t Turn On
Common Causes:
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Thermostat issues
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Tripped circuit breaker
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Blown fuse
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Dirty filter
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Faulty ignition or pilot light
DIY Fixes:
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Check your thermostat: Make sure it’s set to “HEAT” and the temperature is above room temp. Replace batteries if needed.
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Inspect the circuit breaker: Reset it if it’s tripped. If it trips again, call a pro.
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Replace a dirty filter: Restricted airflow can cause the system to overheat and shut down.
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Check the power switch: Furnaces often have a wall switch nearby that looks like a light switch—make sure it’s ON.
Call a Pro If:
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The blower runs but there’s no heat
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You hear clicking but the furnace doesn’t ignite
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The pilot light won’t stay lit
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Electrical issues persist
2. Furnace Turns On but No Heat Comes Out
Common Causes:
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Dirty or clogged air filter
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Faulty burners
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Pilot light or electronic ignition failure
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Gas valve issue
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Blower motor malfunction
DIY Fixes:
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Replace the air filter
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Reset the system: Turn the unit off for 30 seconds, then restart.
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Check for airflow at vents—make sure they’re not blocked.
Call a Pro If:
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The burners won’t ignite
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You hear booming or rumbling noises (this may indicate delayed ignition)
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You smell gas or hear a hissing sound
According to Energy Star, clogged burners and ignition issues are common in neglected furnaces and require professional cleaning and adjustment.
3. Furnace Keeps Shutting Off (Short Cycling)
Common Causes:
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Thermostat location (too close to heat source)
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Dirty flame sensor
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Oversized furnace
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Blocked exhaust vent
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Clogged filter
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Improper airflow
DIY Fixes:
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Change the air filter
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Inspect vents and registers for obstructions
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Check the thermostat for drafts or heat sources nearby
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Clean the flame sensor (if accessible and you’re comfortable doing so)
Call a Pro If:
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Cleaning the flame sensor doesn’t help
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Your system repeatedly turns on/off in minutes
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You suspect the furnace is too large for your home
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There’s moisture or rust around the unit
The U.S. Department of Energy warns that short cycling not only reduces efficiency but puts excess wear on system components.
4. Cold Air Coming from Vents
Common Causes:
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Fan setting on thermostat set to “ON” instead of “AUTO”
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Furnace overheated and shut off the burners
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Dirty flame sensor
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Malfunctioning control board
DIY Fixes:
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Switch thermostat fan to AUTO
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Check filter and replace if dirty
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Reset the furnace by turning it off at the breaker and restarting after a few minutes
Call a Pro If:
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The issue persists with a clean filter and reset
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The blower runs constantly
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The burners never ignite
5. Strange Noises: Banging, Squealing, Rattling
Common Causes:
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Loose panels or ductwork
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Worn blower belt or motor bearings
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Dirty burners causing delayed ignition
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Expansion of ductwork
DIY Fixes:
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Tighten access panels
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Check ductwork for loose sections
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Lubricate blower motor (if your model requires it—many newer ones are sealed)
Call a Pro If:
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You hear loud banging or booming
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Squealing persists even after lubrication
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You suspect blower motor damage
The Home Energy Saver project from Lawrence Berkeley National Laboratory notes that noisy operation is often the first sign of mechanical failure.
6. Pilot Light or Ignition Problems
Common Causes:
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Dirty pilot or flame sensor
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Thermocouple failure
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Drafts blowing out the pilot
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Faulty electronic ignition (spark or hot surface)
DIY Fixes:
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Relight the pilot light (if your model uses one and you're comfortable doing so—always follow the manual)
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Gently clean the flame sensor with steel wool if accessible
Call a Pro If:
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The pilot won’t stay lit
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The ignition module clicks without lighting
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You smell gas
7. Blower Won’t Stop Running
Common Causes:
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Fan set to “ON” at thermostat
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Faulty limit switch
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Blower relay stuck closed
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Thermostat malfunction
DIY Fixes:
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Switch fan to AUTO on the thermostat
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Try a thermostat reset or battery replacement
Call a Pro If:
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The blower continues running regardless of settings
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You hear humming or buzzing
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The unit runs continuously without heating
8. Unusual Smells
Types of Odors and What They Mean:
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Burning Dust – Common after first use of the season; clears up in a few hours
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Electrical or Metallic Smell – May indicate overheating or wiring issues
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Rotten Egg Smell – Indicates a natural gas leak—evacuate and call 911 immediately
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Moldy or Musty Smell – Often from dirty ducts or humid air handler
The Environmental Protection Agency (EPA) explains that indoor air quality can be significantly affected by HVAC performance and filtration.
DIY Fixes:
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Clean or replace air filter
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Vacuum around the furnace
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Ensure return vents are clear
Call a Pro If:
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The odor is persistent
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You detect any chemical or gas smell
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Odors worsen when the furnace turns on
When It’s Time to Call a Pro
There’s no shame in picking up the phone when the problem is beyond your toolbox. Here’s when you should definitely leave it to a professional:
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Frequent shutdowns or restarts
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Gas or electrical smells
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Water pooling near the furnace
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You’ve tried all basic fixes with no improvement
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You’re unsure about opening any components
Not only can licensed HVAC technicians diagnose deeper issues faster, but they can also protect your manufacturer’s warranty by ensuring repairs are done to spec. Certified pros also have tools to test combustion safety and duct pressures—things homeowners can’t safely do on their own.
Maintenance to Prevent Problems
Most furnace issues stem from neglect. Here are a few habits that can help you avoid most of the problems in this list:
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Replace filters monthly
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Schedule an annual professional inspection
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Keep the area around your furnace clean and clear
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Use a programmable thermostat to reduce wear
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Pay attention to sounds and smells
Thinking About a Replacement?
If your furnace is more than 15 years old and starting to rack up service calls, it might be time to consider a replacement. Newer models offer:
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Higher efficiency (up to 98% AFUE)
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Quieter operation
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Smarter thermostats
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Lower emissions
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Better compatibility with R-32 refrigerant AC systems
When you're ready to upgrade, shop reliable, high-efficiency models at The Furnace Outlet. You’ll find options for every budget and home size.
Final Thoughts
Don’t let a furnace hiccup freeze your comfort. With the right mix of DIY troubleshooting and knowing when to call a pro, you can avoid most breakdowns, save money, and keep your system running smoothly all winter long.
And remember, the goal isn’t just a warm house—it’s peace of mind. Stay safe, stay warm, and take care of your system so it can take care of you.
Mark Callahan
HVAC Expert & Home Comfort Advisor
