Furnace Not Working? 9 Common Causes and What to Do

5-Minute Checklist

• Thermostat set correctly? Mode = Heat, setpoint above current room temperature, batteries fresh.
• All supply and return vents open? Closed vents in multiple rooms can cause the system to overheat and shut off on a safety limit.
• Furnace power switch on? Looks like a standard light switch, usually on the unit or nearby wall.
• Furnace door fully closed? Safety interlock cuts power when the access panel is ajar.
• Circuit breaker not tripped? Check the panel.

The 9 Most Common Causes

1. Tripped Circuit Breaker — DIY

Gas furnaces still run on electricity for the igniter, blower, and controls. Reset the breaker once; if it trips again, call a technician. Repair cost if recurring: $150–$400.

2. Dirty or Clogged Air Filter — DIY

A severely clogged filter causes the furnace to overheat and shut off on its high-limit switch. Replace the filter. Most 1-inch filters need replacing every 60–90 days; 4-inch pleated filters last 6–9 months. See the HVAC Maintenance Schedule for full filter intervals. Cost: $5–$30.

3. Ignition Failure (Igniter or Flame Sensor) — PRO

Modern gas furnaces use a hot surface igniter or electronic spark. If the igniter fails or the flame sensor is coated with oxidation, the furnace locks out. Signs: furnace cycles but no heat follows. Repair cost: $150–$400 for igniter; $75–$200 for flame sensor.

4. Pilot Light Out (Older Furnaces) — DIY for relight, PRO if recurring

Furnaces older than ~15 years may have a standing pilot light. Relight following the label instructions. If it won't stay lit, the thermocouple is likely worn out. Cost: $75–$200.

5. Condensate Drain Clog (High-Efficiency Furnaces) — DIY attempt, then PRO

High-efficiency (90%+ AFUE) furnaces produce condensate that drains through PVC pipe. A clogged drain triggers a safety switch shutoff. Flush the drain line with warm water or use a wet-dry vac. Cost if pro needed: $75–$250.

6. Gas Supply Issue — Check, then Utility or PRO

No gas = no heat. Check that other gas appliances work. Confirm the gas shutoff valve near the furnace is open (handle parallel to the pipe). If the gas valve on the furnace has failed: $300–$600 repair.

7. Draft Inducer Motor Failure — PRO

The draft inducer fan pulls combustion gases through the heat exchanger before the burner lights. Without it running, the furnace will not fire. Signs: grinding or squealing before shutdown. Repair cost: $400–$900.

8. Blower Motor Failure — PRO

The blower circulates warm air through ducts. If it fails, the furnace overheats and shuts off. Signs: furnace lights briefly, warm air comes, then the system shuts off. See the HVAC Cost Guide for full repair vs. replacement cost context. Repair cost: $400–$1,200.

9. Cracked Heat Exchanger — PRO IMMEDIATELY

The most serious furnace problem. The heat exchanger separates combustion gases (including CO) from household air. A crack can allow CO to enter the airstream. Signs: unusual smell when furnace runs, CO detector alarm, technician report after inspection. Do NOT run the furnace with a cracked heat exchanger. Make sure CO detectors are working on every floor of your home.

If you need emergency help, see our Emergency HVAC Repair Guide for what to do and what to expect after hours.

Quick Reference: All 9 Causes at a Glance

When to Repair vs. Replace Your Furnace

If replacement is on the table, read our Best Time to Replace Your HVAC System guide before committing — timing your purchase can save hundreds. Check HVAC Financing Options if the cost of a new furnace is a barrier, and review the HVAC Warranty Guide before signing any contract.

Frequently Asked Questions

My furnace is blowing cold air. Is that the same problem?

Not necessarily. A furnace blowing cold air is usually still running — the burner may not be lighting (igniter/flame sensor, pilot, gas supply issues), or the system may be running in fan-only mode (check thermostat fan setting — it should be 'auto,' not 'on'). If the burner is lighting but the air feels barely warm, check the filter first, then look at blower motor issues.

How many times should I let the furnace try to restart before calling?

Most modern furnaces attempt ignition 2–3 times, then lock out and flash an error code. Let it go through that sequence once to capture the error code. Reset the furnace (thermostat to 'off,' wait 30 seconds, back to 'heat') and let it attempt once more. If it fails again, note the error code and call a technician. Repeated cycling without diagnosis can wear the igniter.

The furnace error code is flashing. How do I decode it?

The pattern (e.g., 3 flashes, pause, 1 flash = code 31) is decoded on a label inside the furnace access panel door. Look for a sticker with a code chart — every major manufacturer uses them. Common codes include: ignition failure, pressure switch error, limit switch trip, and flame sensor fault. Note the code before calling a technician.

My furnace runs but the house won't get above 60°F on very cold days. Is something broken?

Possibly not. A furnace is designed to maintain setpoint against your region's design temperature — the coldest conditions your area experiences a few times per season. On extreme cold days below design temperature, even a correctly functioning furnace may struggle to hold setpoint. Check: Is the filter clean? Are all vents open? If the system hasn't been tuned up recently, a technician can verify it's operating at full capacity.

Is it safe to use space heaters while I wait for the furnace to be repaired?

Electric space heaters are safe for temporary use — follow manufacturer guidelines, keep them away from combustibles, and don't run them unattended. Do NOT use gas-burning space heaters (propane or natural gas) indoors without proper ventilation — they produce CO and moisture.

How urgent is furnace repair in winter?

It depends on outdoor temperatures and your home's thermal mass. In mild conditions (40°F+ outdoors), a well-insulated home may have 12–24 hours before temperatures drop uncomfortably. In extreme cold (below 0°F outdoors), pipes can freeze and burst when indoor temperatures drop below 32°F in the walls. If temperatures are extreme, treat this as an emergency.

My furnace is making a banging noise when it starts. Is that serious?

A loud bang at startup — called delayed ignition — is serious. It means gas is building up in the combustion chamber before the igniter fires, then igniting in a small explosion. Over time this can crack the heat exchanger. Stop using the furnace and call a technician.

When should I replace rather than repair my furnace?

Replace when: the furnace is 15+ years old, repair costs approach or exceed $1,500 on an aging unit, you've had two or more significant repairs in the past three years, or the heat exchanger is cracked. Repair when: the system is under 15 years old, the repair costs less than 50% of a new system, and the heat exchanger is intact.