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.

Can I run my furnace with the access panel off?

No. The safety interlock switch cuts power when the panel is open. It's there to prevent operating the furnace with combustion air and blower access exposed. Some homeowners tape or bypass this switch — don't. If you're observing the burner for diagnostic purposes, observe briefly with the panel held in place and the switch depressed manually, then close it before extended operation.

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. Kerosene heaters require ventilation and are a fire risk indoors.

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.

How long does a furnace repair usually take?

Simple repairs (filter, igniter, flame sensor, thermocouple) often take 30–90 minutes once the technician arrives. More complex repairs (inducer motor, gas valve, control board) may take 2–4 hours or require a return trip if the part isn't on the truck. Heat exchanger replacement is a multi-hour job — often 4–8 hours — and some manufacturers require factory-authorized technicians to maintain the warranty.

Homeowner Guide

Furnace Not Working? 9 Common Causes and What to Do (Winter Troubleshooting Guide)

Last updated: July 2026 · HVACListing.com Editorial

If you smell gas — leave immediately and call the gas utility emergency line from outside. Do not touch light switches, phones, or thermostats. No gas smell? The 9 causes below cover 95% of furnace failures.

Your furnace stopped working. It's cold. Before you call anyone, there are three things you should do in the first five minutes — and several of them might solve the problem entirely.

If there's no gas smell, proceed through the causes below in order. They're ranked from most common and easiest to fix, to least common and most serious.

Before anything else: the 5-minute checklist

Run through these before assuming a repair is needed:

Thermostat set correctly? Mode should be "Heat," not "Cool" or "Off." Temperature setpoint should be above the current room temperature. Check that it didn't get bumped or that a schedule change pushed the setpoint down.
Thermostat batteries? Many programmable and smart thermostats lose connection or revert to a default setpoint (often 65°F) when batteries die. Swap them.
All supply and return vents open? Closed vents in multiple rooms can cause the system to overheat and shut off on a safety limit. Open them all, including in unused rooms.
Furnace power switch on? The furnace has a dedicated power switch — it looks like a standard light switch, usually on the unit or nearby wall. It sometimes gets switched off accidentally.
Furnace door fully closed? Most furnaces have a safety interlock that cuts power when the access panel is ajar. Push it closed firmly.

If none of these solve it, work through the causes below.

The 9 most common reasons a furnace stops working

1. Tripped circuit breaker — DIY

Frequency: Very common

Gas furnaces still run on electricity — for the igniter, blower motor, controls, and inducer fan. If the breaker labeled "furnace" or "air handler" tripped, the furnace won't fire even though gas is available.

What to do: Find the breaker in your electrical panel. If it's tripped (center position or fully off), flip it fully off and then back on. Watch whether it trips again immediately — if it does, stop and call an electrician or HVAC technician. A breaker that trips repeatedly is protecting against a real electrical problem; resetting it repeatedly can cause damage.

Cost if it recurs: $150–$400 for an electrician or HVAC tech to diagnose the underlying cause.

2. Dirty or clogged air filter — DIY

Frequency: Very common

A severely clogged filter restricts airflow so severely that the furnace overheats and shuts off on its high-limit safety switch. The system may try to restart and shut off again immediately, or fail to start at all.

What to do: Find the filter (usually in the return air grille or at the air handler). If it's gray and visibly clogged, replace it. Run the system and see if it comes back. Most 1-inch filters should be replaced every 60–90 days; 4-inch pleated filters last 6–9 months. See the HVAC Maintenance Schedule for full filter intervals.

Cost: $5–$30 for a replacement filter.

Prevention: Set a recurring reminder. A dirty filter is the single most preventable furnace failure.

3. Ignition failure (igniter or flame sensor) — PRO

Frequency: Common

Modern gas furnaces use either a hot surface igniter (a glowing element, like a miniature electric stove burner) or an electronic spark igniter to light the burner. Both can fail. The furnace will attempt to light, fail, try again, then lock out.

Signs: You hear the furnace cycle — the inducer fan starts, there's a click or hum — but no heat follows. After 2–3 attempts, the furnace locks out (often indicated by a flashing error code on the control board).

A dirty or failing flame sensor is equally common: the flame sensor is a small metal rod that detects whether the burner is actually lit. If it's coated with oxidation, it can't confirm ignition and the gas valve closes as a safety measure, even though the flame was lit.

What to do: Note the error code on the furnace's control board (a sequence of LED flashes). Look up the code in the manual or on the label inside the furnace door — it will confirm ignition failure. Call a technician to replace the igniter or clean/replace the flame sensor.

Repair cost: $150–$400 for igniter replacement; $75–$200 for flame sensor cleaning or replacement.

4. Pilot light out (older furnaces) — DIY for relight, PRO if recurring

Frequency: Common in furnaces older than ~15 years

Furnaces manufactured before approximately 2010 often use a standing pilot light rather than electronic ignition. If the pilot goes out, the furnace won't fire.

What to do: Locate the pilot assembly (usually visible through an observation window). Follow the relight instructions on the furnace label — typically: set the gas valve to "Pilot," hold the button while lighting with a long lighter, continue holding for 30 seconds after the flame catches, then release and turn the valve to "On."

If the pilot won't stay lit or goes out repeatedly, the thermocouple (a safety device that keeps the gas valve open when the pilot is burning) is likely worn out. Thermocouple replacement is a $20 part and a 30-minute job for an HVAC technician.

Repair cost: $75–$200 to replace a thermocouple.

Note: A furnace old enough to have a standing pilot light is likely 15–25+ years old. Factor replacement into your planning. See Best Time to Replace Your HVAC System for timing and cost guidance.

5. Condensate drain clog (high-efficiency furnaces) — DIY attempt, then PRO

Frequency: Moderate

High-efficiency (90%+ AFUE) furnaces extract so much heat from combustion gases that the exhaust condenses into water. This condensate drains through a PVC pipe into a floor drain or condensate pump. If that drain clogs, a safety switch shuts the furnace off.

Signs: A small amount of water near the furnace base, or the furnace not starting at all despite no error codes on the ignition sequence.

What to do: Locate the condensate drain line and the safety switch (usually a small float switch in a trap or reservoir). Clear the drain line by flushing with warm water or using a wet-dry vacuum. If the float switch is stuck or the pump is seized, call a technician.

Repair cost: $75–$250 for professional drain clear or condensate pump replacement.

6. Gas supply issue — Check, then utility or PRO

Frequency: Moderate

No gas supply = no heat, regardless of what else is working.

What to do:

Check that your gas bill is paid and service is active.
Locate the gas shutoff valve on the gas line leading to the furnace (usually a lever or ball valve within a few feet of the unit). Confirm it's in the open position — handle parallel to the pipe means open; perpendicular means closed.
Check whether other gas appliances in the home (stove, water heater) are working. If they're also out, contact your gas utility — there may be a neighborhood outage or a problem at the meter.

If gas supply is confirmed but the furnace still won't fire, the gas valve on the furnace itself may have failed — a technician repair.

Repair cost: $300–$600 for a gas valve replacement.

7. Draft inducer motor failure — PRO

Frequency: Less common

The draft inducer motor (also called the inducer fan or vent fan) pulls combustion gases through the heat exchanger and out the flue before the burner lights. It's a safety component — the furnace will not light without it running.

Signs: The furnace tries to start but you hear no fan spinning, or a grinding/squealing noise before shutdown. The pressure switch (which confirms the inducer is creating adequate draft) trips as a result.

What to do: Call a technician. Do not attempt to bypass the pressure switch — it's a safety interlock.

Repair cost: $400–$900 for inducer motor and labor.

8. Blower motor failure — PRO

Frequency: Less common, more serious

The blower motor circulates warm air through the duct system. If it fails, the heat exchanger overheats rapidly and the furnace shuts off on its high-limit switch. The burner fires but no warm air moves through the house.

Signs: You hear the furnace light (burner fires), warm air briefly comes out, then the furnace shuts off. The cycle may repeat. Eventually the high-limit trips and the furnace locks out.

What to do: Call a technician. Blower motor failure can be confirmed by a technician measuring motor amperage or checking for a locked shaft. See the HVAC Cost Guide for full repair vs. replacement cost context.

Repair cost: $400–$1,200 depending on motor type (PSC vs. ECM variable-speed motors cost more).

9. Cracked heat exchanger — PRO IMMEDIATELY, consider replacement

Frequency: Less common, most serious

The heat exchanger is the metal component that separates combustion gases (including carbon monoxide) from the air your family breathes. If it cracks, CO can enter the airstream.

This is the most serious furnace problem. It typically triggers the furnace's safety controls to shut the unit down — but not always.

Signs: A strong smell when the furnace runs (combustion gases mixing with indoor air), unexplained headaches or nausea in the household, CO detector alarm, or a technician's report after inspection.

What to do:

If your CO detector is alarming: evacuate immediately, leave the door open, call 911 from outside.
If a technician finds a cracked heat exchanger during inspection: do not run the furnace until it's repaired or replaced.

Repair cost: $1,000–$3,500 for heat exchanger replacement — if the part is available and the furnace is worth repairing. A cracked heat exchanger in a furnace older than 10–15 years is almost always a replacement decision, not a repair decision. The rest of the furnace has similar wear, and the repair cost often exceeds what the system is worth.

Make sure you have working CO detectors on every level of your home. Per NFPA 720, they should be installed on each floor including the basement. Test them monthly.

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

Quick reference: causes ranked by urgency

Cause	DIY or PRO	Typical Repair Cost	Urgency
Tripped breaker	DIY (reset once)	$0 / $150–$400 if recurring	Low
Dirty filter	DIY	$5–$30	Low
Igniter / flame sensor	PRO	$150–$400	Medium
Pilot light out	DIY relight / PRO recurring	$0 / $75–$200	Medium
Condensate drain clog	DIY attempt	$75–$250	Medium
Gas supply issue	Check / utility / PRO	$300–$600 (gas valve)	Medium–High
Draft inducer motor	PRO	$400–$900	High
Blower motor	PRO	$400–$1,200	High
Cracked heat exchanger	PRO — do not run	$1,000–$3,500 or replace	Immediate

When to repair vs. replace

Repair when:

The system is under 15 years old
Repair costs less than 50% of a comparable new system
The heat exchanger is intact
The system has been well-maintained with no pattern of recurring failures

Replace when:

The furnace is 15–25+ years old (average lifespan is 15–20 years)
Repair costs approach or exceed $1,500 on an aging unit
Two or more significant repairs in the past three years
The heat exchanger is cracked
The system runs at dramatically lower AFUE than modern units

A new high-efficiency furnace (95–98% AFUE) in a home currently running a 70% AFUE unit cuts fuel consumption for heating by roughly 25–30%. In colder climates like Denver, that's meaningful real-world savings year over year.

See our HVAC Cost Guide for full replacement cost ranges and our Best Time to Replace Your HVAC System for timing and off-season savings guidance. If financing a replacement is a concern, our HVAC Financing Options Guide covers the full range from manufacturer programs to HELOCs. And before signing any contract, review the HVAC Warranty Guide so you know what's covered.

What to tell a technician when you call

When you call for emergency or next-day service, having this information ready helps the technician arrive prepared with the right parts:

Furnace brand and model number (on a label inside the access panel door)
Age of the system (installation date, often on the label; or check the serial number — most manufacturers encode the manufacturing year in it)
What the furnace is doing: Is it completely silent? Starting then stopping? Clicking but not lighting? Running but not heating?
Any error codes visible on the control board (sequence of LED flashes)
Whether you smell anything — gas, burning, unusual odors
Whether the problem is intermittent or constant

A prepared technician call saves diagnostic time and may let them arrive with the correct part for a one-trip repair. See our How to Hire an HVAC Contractor guide for what to verify before you commit to a service company — especially for emergency calls where you have less time to vet.

Keeping your furnace running through winter

Most furnace failures in winter could have been prevented — or caught early — with fall maintenance:

Replace the filter before heating season (September or October)
Schedule a furnace tune-up in September or October — before the technician's schedule fills in November. A tune-up ($75–$200) includes: heat exchanger visual inspection, burner cleaning, igniter check, gas pressure measurement, flue draft test, blower amperage measurement, and condensate drain clear on high-efficiency units
Test your CO detectors and replace batteries annually
Clear the area around the furnace — combustibles within 3 feet of the unit are a fire risk and can also block combustion air intake
Know where your gas shutoff is before you need it

Our HVAC Maintenance Schedule covers the full seasonal checklist for both homeowner and professional tasks.

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 (causes #3–6 above), or the system may be running in "fan only" mode (check the 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 issues (#8).
Can I run my furnace with the access panel off?: No. The safety interlock switch cuts power when the panel is open. It's there to prevent operating the furnace with combustion air and blower access exposed. Some homeowners tape or bypass this switch — don't. If you're observing the burner for diagnostic purposes, observe briefly with the panel held in place and the switch depressed manually, then close it before extended operation.
How many times should I let the furnace try to restart before calling?: Most modern furnaces will 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. Then reset the furnace (turn the thermostat to "off," wait 30 seconds, turn it 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 a set temperature against a specific outdoor design temperature — typically the coldest conditions your region 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. Kerosene heaters require ventilation and are a fire risk indoors.
How urgent is furnace repair in winter?: It depends on outdoor temperatures and your home's thermal mass. In mild winter conditions (40°F+ outdoors), a well-insulated home will lose heat slowly — you likely have 12–24 hours before the indoor temperature drops to uncomfortable levels. In a polar vortex situation (-10°F outdoors), a poorly insulated home can drop to freezing within a few hours. 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 or "boom" 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. This is not a noise-only problem — it's a mechanical and safety issue that requires diagnosis.
How long does a furnace repair usually take?: Simple repairs (filter, igniter, flame sensor, thermocouple) often take 30–90 minutes once the technician arrives. More complex repairs (inducer motor, gas valve, control board) may take 2–4 hours or require a return trip if the part isn't on the truck. Heat exchanger replacement is a multi-hour job — often 4–8 hours — and some manufacturers require factory-authorized technicians to maintain the warranty.

Find licensed heating contractors near you

Search our directory for furnace repair specialists in your city.

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Related guides

Sources

NFPA 720: Standard for the Installation of Carbon Monoxide (CO) Detection and Warning Equipment — National Fire Protection Association
ACCA Manual J: Residential Load Calculation (8th edition) — design temperature references
U.S. Department of Energy: Furnace efficiency (AFUE) standards and definitions
EPA ENERGY STAR: Minimum efficiency thresholds for ENERGY STAR-certified gas furnaces (97% AFUE for northern regions as of 2023)
Gas Technology Institute: Residential gas appliance safety guidance
Building Science Corporation: Heat exchanger failure modes and CO risk analysis

Cost ranges aggregated from Atlanta, Tampa, and Denver markets, July 2026. Individual quotes will vary by region, equipment brand, and labor rates. Always obtain at least two estimates for repairs exceeding $400.