10 Most Common Furnace Problems (And What They Cost to Fix)

Most furnace breakdowns trace back to the same handful of issues. Understanding these problems helps you decide what you can handle yourself and when to call a professional.

1. Dirty or Clogged Air Filter ($5–$30)

What it is: The air filter traps dust, pet hair, and debris before air enters the furnace. When it gets clogged, airflow is restricted, forcing the furnace to work harder and potentially overheat.

Symptoms:

• Weak airflow from vents
• Furnace runs constantly but doesn't reach set temperature
• Furnace short cycles (turns on and off frequently)
• Higher-than-normal energy bills

Typical cost to fix: $5–$30 for a replacement filter.

DIY vs. pro: This is the easiest furnace fix and should always be a DIY task. Check your filter monthly and replace it every 1–3 months depending on your home (more often if you have pets or allergies). A clogged filter is the cause of roughly 80% of furnace service calls that could have been avoided.

2. Thermostat Issues ($50–$400)

What it is: The thermostat tells your furnace when to turn on and off. Problems range from dead batteries to faulty wiring to a thermostat that's simply worn out.

Symptoms:

• Furnace doesn't turn on at all
• Temperature doesn't match the thermostat setting
• Furnace cycles erratically
• Blank thermostat display

Typical cost to fix: $50–$150 for a repair; $100–$400 for a new thermostat installed.

DIY vs. pro: Start with the basics: replace batteries, check that it's set to "heat" mode, and make sure the temperature is set above the current room temperature. If those don't solve it, a pro can diagnose wiring issues or install a replacement.

3. Ignition or Pilot Light Problems ($100–$400)

What it is: Modern furnaces use electronic ignition (hot surface ignitor or spark ignitor), while older units have a standing pilot light. When ignition fails, the furnace can't light the burners.

Symptoms:

• Furnace clicks but doesn't ignite
• Pilot light won't stay lit
• Blower runs but no heat is produced
• Furnace locks out after failed ignition attempts

Typical cost to fix: $100–$400 for ignitor replacement; $100–$300 for thermocouple replacement on older pilot-light systems.

DIY vs. pro: Relighting a pilot light is a basic DIY task (follow your furnace manual). Replacing an ignitor or thermocouple requires some mechanical skill — most homeowners should call a pro for safety reasons.

4. Dirty Flame Sensor ($75–$300)

What it is: The flame sensor is a small metal rod that detects whether the burners are actually producing a flame. Carbon buildup on the sensor prevents it from reading correctly, causing the furnace to shut down as a safety precaution.

Symptoms:

• Furnace starts, runs for a few seconds, then shuts off
• Repeated start-stop cycling
• Furnace locks out with a flashing error code

Typical cost to fix: $75–$150 for cleaning; $150–$300 for replacement.

DIY vs. pro: Cleaning a flame sensor is a relatively simple job — remove the sensor, gently scrub it with fine steel wool or emery cloth, and reinstall. However, if you're not comfortable working around gas appliances, call a technician. This is the single most common furnace repair call.

5. Cracked Heat Exchanger ($500–$3,500)

What it is: The heat exchanger is a set of metal coils or tubes that transfer heat from combustion gases to the air circulating through your home. Over time, repeated heating and cooling cycles cause metal fatigue and cracking.

Symptoms:

• Yellow or flickering burner flame (should be steady blue)
• Strong, unusual odor when the furnace runs
• Soot buildup inside the furnace
• Carbon monoxide detector alarms
• Water pooling around the furnace base

Typical cost to fix: $500–$3,500 for replacement.

DIY vs. pro: This is always a professional repair. A cracked heat exchanger is a serious safety hazard — it can leak carbon monoxide into your home. For furnaces over 15 years old, replacement of the entire unit is usually more cost-effective than replacing the heat exchanger alone. Use the $5,000 rule to decide whether to repair or replace.

6. Blower Motor Failure ($300–$2,500)

What it is: The blower motor drives the fan that pushes heated air through your ductwork. Motors wear out over time, especially if they've been running with restricted airflow from dirty filters.

Symptoms:

• No airflow from vents even though the furnace sounds like it's running
• Weak or inconsistent airflow
• Loud humming, squealing, or grinding noises
• Furnace overheats and shuts down
• Burning smell

Typical cost to fix: $300–$800 for single-speed motors; $800–$2,500 for variable-speed motors.

DIY vs. pro: Always a professional repair. Blower motors involve electrical work and proper sizing.

7. Unusual Noises ($100–$1,500)

What it is: Different furnace noises point to different problems. The sound itself is a useful diagnostic clue.

Common noises and causes:

• Banging or booming: Delayed ignition (gas builds up before lighting), dirty burners, or expanding/contracting ductwork. Cost: $100–$400.
• Squealing or screeching: Worn blower motor belt, failing motor bearings, or shaft misalignment. Cost: $100–$300 for belt; $300–$2,500 for motor.
• Rattling: Loose screws, panels, or ductwork connections. Sometimes indicates a cracked heat exchanger. Cost: $0–$50 for loose panels; $500–$3,500 for heat exchanger.
• Clicking: Normal at startup/shutdown, but persistent clicking suggests a failing ignitor or control board issue. Cost: $100–$600.
• Humming: Normal at low volume; loud humming suggests a failing blower motor capacitor ($100–$300) or transformer ($75–$200).

DIY vs. pro: Tightening loose panels and screws is DIY. Any noise that's new, loud, or persistent warrants a professional diagnosis.

8. Short Cycling ($100–$600)

What it is: Short cycling means the furnace turns on and off rapidly — running for only a few minutes before shutting down, then restarting. This wastes energy, causes uneven heating, and accelerates wear on components.

Common causes:

• Clogged air filter (most common — replace the filter first)
• Oversized furnace for the home
• Faulty flame sensor
• Overheating due to restricted airflow
• Thermostat issues (placement near a heat source, or malfunction)

Typical cost to fix: $5–$30 if it's just a filter; $100–$600 for other causes.

DIY vs. pro: Replace the filter first. If short cycling continues, call a pro to check the flame sensor, thermostat, and airflow.

9. Yellow Burner Flame ($100–$500)

What it is: A properly operating gas furnace should produce a steady, blue flame. A yellow or orange flame indicates incomplete combustion — the gas isn't burning cleanly.

Causes:

• Dirty burners
• Incorrect gas pressure
• Cracked heat exchanger (serious)
• Insufficient air supply for combustion

Typical cost to fix: $100–$300 for burner cleaning and adjustment; $500–$3,500 if a cracked heat exchanger is the cause.

DIY vs. pro: Always call a professional. Yellow flames can indicate carbon monoxide production, which is a health and safety risk. If this is happening outside business hours, see our emergency furnace repair guide.

10. Draft Inducer Motor Problems ($200–$1,500)

What it is: The draft inducer motor pulls combustion gases through the heat exchanger and exhausts them out the flue. When it fails, the furnace won't start because the pressure switch can't verify proper venting.

Symptoms:

• Furnace won't start
• Loud humming or whining before the burners ignite
• Pressure switch error codes
• Vibrations from the furnace cabinet

Typical cost to fix: $200–$1,500 depending on the motor and furnace model.

DIY vs. pro: Always a professional repair. The draft inducer is critical to safe venting of combustion gases.