10 Signs Your Furnace Is About to Fail (and What to Do Next)

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Furnaces rarely die without warning. In most cases, they send signals for weeks or months before they quit: strange noises, uneven heat, higher bills. Catch those signs early and you can often fix a small problem before it becomes a no-heat emergency on the coldest night of the year.

Here are the 10 warning signs to watch for, what each one usually means, and when it's time to call a pro.

⚠️ Safety first: If you smell gas (a rotten-egg smell) or your carbon monoxide alarm goes off, leave the house immediately and call your gas company or 911 from outside. Don't flip switches or use your phone inside.

1. Your furnace is more than 15 years old

Age alone isn't a problem, but a gas furnace typically lasts 15 to 20 years. Once yours is in that range, repairs get more frequent and parts get harder to find. If your furnace is 15+ years old and showing any other sign on this list, it's smart to start planning for a replacement.

Tip: Look for the manufacture date on the label inside the furnace's access panel.

2. Your heating bills keep going up

If your bills are rising faster than energy prices, and you haven't changed how you use your heat, your furnace may be losing efficiency. Worn parts, dirty burners and failing components make a furnace run longer to deliver the same heat.

3. Strange noises

Furnaces make some noise, but new or loud sounds usually mean something's wrong:

Noise	What it might mean
Banging or booming at startup	Delayed ignition: gas building up before it lights. Call a pro.
Squealing or screeching	A worn blower belt or failing blower motor bearings
Rattling	Loose panels, screws or ductwork, or, more seriously, a cracked heat exchanger
Clicking that doesn't stop	An ignition problem or failing control
Humming louder than usual	Blower motor or electrical issue

4. It turns on and off too often (short cycling)

If your furnace runs for only a few minutes, shuts off, then starts again, it's short cycling. Common causes include a clogged filter, a dirty flame sensor, a thermostat problem, an overheating furnace or a furnace that's too big for your home. Short cycling wastes energy and wears out parts quickly.

5. Some rooms are warm, others are cold

Uneven heating can come from blocked vents or leaky ducts, but it can also mean your furnace can no longer push enough warm air through the house, a sign of a weakening blower or a system that's struggling.

6. A yellow or flickering burner flame

On a gas furnace, the burner flame should be steady and blue. A yellow, orange or flickering flame can mean incomplete combustion, which can produce carbon monoxide. Have it checked by a professional right away.

7. Carbon monoxide warning signs

Watch for:

Your CO alarm going off
Headaches, dizziness, nausea or flu-like symptoms that improve when you leave the house
Soot or rust around the furnace or vents
Excess moisture on windows near the furnace

A cracked heat exchanger is one of the most serious furnace problems because it can let carbon monoxide into your home. If you notice any of these signs, get everyone out and call for help.

8. Unusual smells

Dusty burning smell for a few minutes at the first startup of the season: usually normal (dust burning off).
Burning plastic or electrical smell: turn the furnace off and call a technician.
Rotten eggs (gas): leave the house and call your gas company immediately.

9. The furnace keeps needing repairs

Two or more significant repairs in the last two years is a strong sign your furnace is near the end. At some point, putting more money into an old system doesn't make sense.

10. Your home feels dry, dusty or stuffy

An aging furnace can struggle to circulate and filter air properly. More dust, stuffiness or very dry air can point to problems with the blower, ductwork or filter.

Quick checks to try before calling a technician

Many "broken furnace" calls turn out to be simple fixes. Before you call, check:

☐ Thermostat: set to Heat, set a few degrees above room temperature, and fresh batteries.
☐ Furnace filter: a badly clogged filter can make the furnace overheat and shut down. Replace it.
☐ Circuit breaker: make sure the furnace breaker hasn't tripped.
☐ Furnace power switch: usually a light switch on or near the furnace. Make sure it's ON.
☐ Furnace door: the front panel must be closed properly, or a safety switch keeps the furnace off.
☐ Vents and returns: not blocked by furniture or rugs.
☐ Outside vent pipes (high-efficiency furnaces): not blocked by snow or ice.
☐ Gas supply: other gas appliances working? Gas valve to the furnace open?

If none of these fixes it, it's time to call a pro. Don't try to repair gas or electrical components yourself.

Typical furnace repair costs

These are rough national ranges for common repairs. Prices vary a lot by location, brand and whether it's an emergency call.

Repair	Typical cost
Service call / diagnosis	$75 to $200
Flame sensor cleaning or replacement	$80 to $250
Igniter replacement	$150 to $450
Thermostat replacement	$150 to $500
Inducer motor	$400 to $1,500
Blower motor	$400 to $1,500
Control board	$300 to $1,000
Heat exchanger replacement	$1,000 to $3,500+

Emergency, nights and weekend calls often cost more. Always ask for the price before work begins.

Repair or replace? A handy rule of thumb: multiply your furnace's age by the repair cost. If it's over $5,000, replacement is usually the better deal.

👉 See what a new one costs: Furnace Replacement Cost in 2026

How to make your furnace last longer

Change the filter every 1 to 3 months during heating season.
Book a professional tune-up every year, ideally in early fall.
Keep the area around the furnace clear: no storage boxes, paint cans or laundry.
Keep vents and returns open and unblocked.
Install a CO detector near the furnace and near bedrooms.
Use a programmable or smart thermostat to reduce run time.

Frequently asked questions

How do I know if my furnace is dying? The most common signs are age (15+ years), rising heating bills, frequent repairs, strange noises, short cycling, uneven heating and a yellow burner flame.

Is a furnace that turns on and off a lot dangerous? Short cycling usually isn't immediately dangerous, but it wastes energy and can be caused by overheating. Change the filter first; if it continues, call a technician.

Why does my furnace smell like burning? A dusty smell at the first startup of the season is normal and should fade quickly. A persistent burning, electrical or plastic smell is not. Turn the furnace off and call a pro.

Can a furnace cause carbon monoxide poisoning? Yes. A cracked heat exchanger, blocked vent or poor combustion can release carbon monoxide. That's why every home with a gas furnace needs working CO alarms.

How much does a furnace inspection cost? A yearly tune-up or inspection typically costs about $80 to $200. Many HVAC companies offer maintenance plans that include an annual visit.

The bottom line

Your furnace usually warns you before it fails. Listen for new noises, watch your bills, look at the flame and pay attention to how evenly your home heats. Try the quick checks first, call a professional for anything involving gas or electrical parts, and if your furnace is 15+ years old and needs a big repair, it may be time to plan for a replacement.

Getting ready for winter? Don't miss our Complete Winterize Your Home Checklist.