Meta Description: Furnace not working? Learn safety checks, quick DIY fixes, error codes, costs, and when to call a pro—so you can restore heat fast and stay safe.
When a furnace stops working on a cold day, it quickly turns urgent. This guide explains how to troubleshoot common causes, what to check first, when it is safe to reset the furnace, and when to call a professional. It also covers costs, timelines, and ways to stay warm and protect your home while waiting for repairs.
Start With Safety
Before touching the thermostat or panel, address safety. If you smell natural gas or hear hissing, evacuate immediately. Do not operate switches or use phones inside. Call your gas utility or 911 from a safe location.
Check that carbon monoxide (CO) detectors are working. There should be CO alarms on each floor and near sleeping areas. Replace batteries if needed. If a CO alarm sounds, evacuate and contact emergency services.
Turn off space heaters near the furnace area. Keep flammables away from the unit. Make sure the furnace has clear access and ventilation.
Quick DIY Fixes For A Furnace Not Working
Thermostat Checks
Set the thermostat to Heat, not Cool or Off. Raise the setpoint 3–5 degrees above room temperature. Ensure the fan is on Auto, not On, to avoid confusing airflow with heat output.
Replace thermostat batteries, even if the display is on. Weak batteries can prevent the call for heat. For smart thermostats, confirm Wi‑Fi and firmware updates are not interrupting calls for heat.
If the home has multiple systems, confirm the thermostat controls the furnace, not a heat pump or AC. Verify schedule settings are not delaying heat with an “Eco” or setback mode.
Power, Switches, And Breakers
Furnaces have a nearby service switch that looks like a light switch. Confirm it is On. Some units have a blower door interlock switch that cuts power when the panel is open; ensure the door is seated tight.
Check the circuit breaker labeled Furnace or HVAC. Reset a tripped breaker by switching it fully Off, then On. If it trips again, stop and call a professional—there may be a short or motor issue.
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Many high-efficiency furnaces have a condensate pump on a GFCI outlet; if that outlet is tripped, the safety float switch may stop the furnace. Reset the GFCI and ensure the pump runs.
Air Filter And Airflow
A clogged filter can trip safeties and cause a “furnace not working” situation. Locate the air filter at the return or in the furnace cabinet. Replace if dirty. Ensure the arrow points toward the furnace.
Open all supply registers and return grills. Avoid blocking them with rugs or furniture. Closed or obstructed vents can cause overheating and shutdowns.
Check the area around the furnace for clutter. Furnaces need clear airflow. Keep at least 3 feet of clearance in front of the unit for service and ventilation.
Gas Supply, Pilot, And Ignition
Verify the gas shutoff valve to the furnace is open. The handle should be parallel to the pipe. If perpendicular, it is closed. Only adjust if it is safe and there is no gas smell.
Most modern gas furnaces use electronic igniters. Older units may have a standing pilot. If the pilot is out, follow the manufacturer’s lighting instructions on the door label. If it will not stay lit, stop and call a pro.
Do not bypass safety switches or hold a pilot open with tools. If ignition fails repeatedly, a flame sensor, igniter, or control issue may be present and needs diagnosis.
Condensate And Outdoor Vents (High-Efficiency Units)
High-efficiency (90%+) furnaces produce water. A blocked condensate drain or full pump can stop operation. Empty the pump reservoir and clear kinks in tubing. Confirm the discharge line is not frozen.
Outside, inspect PVC intake and exhaust pipes. Clear snow, ice, leaves, and nests. Blocked pipes can cause pressure switch faults and shut the furnace down.
Do not use sharp tools inside the pipes. If ice builds repeatedly, a pro should evaluate vent pitch, termination kits, or drainage.
Symptom-Based Troubleshooting Guide
Match what the furnace is doing with the table below to identify likely causes and safe, quick steps to try first.
| Symptom | Likely Cause | Quick Action |
|---|---|---|
| Thermostat Calls, Furnace Silent | Power switch off, tripped breaker, door switch, failed transformer | Check switch, reset breaker once, reseat door. If still silent, call a pro. |
| Starts, Then Shuts Off In Seconds | Flame sensor issue, ignition failure, pressure switch fault | Power cycle once, clean filter, clear intake/exhaust. Call if persists. |
| Blower Runs, No Warm Air | Gas off, igniter failure, closed gas valve, cracked heat exchanger (rare) | Confirm gas valve open, check error code. If gas smell or repeated fault, call immediately. |
| Short Cycling Every Few Minutes | Overheating from clogged filter, closed vents, oversize system | Replace filter, open vents, ensure returns clear. If unresolved, call. |
| Loud Rattling Or Grinding | Blower or inducer bearings, loose panels | Tighten panels, replace filter. If noise continues, shut off and call. |
| Intermittent Operation In Wet Weather | Blocked condensate, frozen discharge line, vent icing | Clear drain, thaw line, remove snow/ice from vents. |
| Repeated Lockouts With LED Code | Control detects unsafe condition | Do not keep resetting. Record code and call a pro. |
When The Furnace Runs But Rooms Stay Cold
Sometimes the furnace fires normally, yet the home is still chilly. That points to airflow or distribution rather than ignition or gas problems.
Inspect ductwork in basements or attics for disconnected runs or large leaks. Metal ducts can separate at elbows. Flexible ducts can kink. Tape-only joints often fail; mastic or proper collars are preferred.
Check that dampers on branch ducts are open. Many have small levers on round ducts. Align the lever with the duct for open. Balance airflow to distant rooms by partially closing nearer runs.
If a central AC evaporator coil sits above the furnace, heavy dust can clog it and restrict airflow. Reduced airflow leads to overheating and shutdowns. Coil cleaning is a professional task.
In very cold snaps, homes may lose heat faster than the furnace can add it. Reduce heat loss with weatherstripping, closing fireplace dampers, and drawing thermal curtains at night.
How To Reset A Furnace Correctly
Many “furnace not working” calls resolve with a safe reset. However, repeated resets can mask unsafe conditions. Follow this one-time procedure.
Step 1: Set the thermostat to Off. If programmable, cancel any hold or schedule.
Step 2: Turn the furnace power switch Off or flip the breaker Off. Wait five minutes to clear soft lockouts and allow the inducer to cool.
Step 3: Restore power, then set the thermostat to Heat and raise the setpoint 5 degrees.
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Step 4: Watch the startup sequence: inducer fan, ignition, gas valve, then main flame, followed by blower. If it fails or locks out again, stop resetting and call a professional.
Never hold an ignition button or bypass a safety switch to “force” operation. Those safeties protect against fire and carbon monoxide.
Reading Furnace Error Codes
Most furnaces have a small viewing window and an LED that blinks error codes. The code chart is often on the inside of the blower door. Note the pattern and number of flashes.
Pressure Switch Open: Often caused by blocked intake/exhaust, cracked or loose rubber tubing, or a failing inducer fan. Clear vents and tubing; if unresolved, service is needed.
Limit Switch Open: Indicates overheating from airflow restriction. Replace a dirty filter, open registers, and check for blocked returns. Persistent faults may indicate a failing blower motor or undersized ductwork.
Ignition Lockout/No Flame Signal: The control board did not sense flame. Causes include a faulty igniter, dirty flame sensor, or gas valve issues. Cleaning and testing typically require a technician.
Rollout Switch Trip: A serious safety condition where flame “rolls out” of the burner area, often due to blocked heat exchangers. Do not reset repeatedly. Call a professional immediately.
Neutral/Polarity Fault: Some furnaces require correct line polarity for proper flame sensing. An electrician may need to correct wiring.
When To Call A Professional
DIY checks are worth trying once. Call a qualified HVAC technician if any of the following occur:
- Gas smell, sooting, or CO alarm activation.
- Breaker trips again after a single reset.
- LED error codes persist after clearing filters and vents.
- Ignition failures, repeated lockouts, or pilot will not stay lit.
- Unusual noises: grinding, screeching, or metal-on-metal.
- Burnt wiring smell or signs of scorching on the control board.
- Water pooling from condensate systems that returns after emptying.
Choose licensed, insured contractors who pull permits when required. Ask about diagnostic fees, warranty coverage, and whether the truck is stocked for same-day repairs.
Typical Repair Costs And Timelines
Actual pricing varies by region, brand, and warranty status. The ranges below are common for U.S. residential furnaces.
| Repair | Typical Cost (Parts + Labor) | Usual Timeline |
|---|---|---|
| Service Call + Diagnosis | $100–$200 | Same day |
| Igniter Replacement | $150–$350 | Same day (in-stock) |
| Flame Sensor Service/Replacement | $80–$250 | Same day |
| Blower Motor (PSC) | $400–$900 | Same day or next day |
| ECM/Variable-Speed Blower Motor | $700–$1,400 | 1–2 days |
| Control Board | $300–$700 | Same day or next day |
| Inducer Motor | $400–$900 | 1–2 days |
| Gas Valve | $300–$600 | 1–2 days |
| Condensate Pump/Drain Repair | $150–$350 | Same day |
| Heat Exchanger Replacement | $1,500–$3,000 | 2–5 days (often warranty parts) |
| New Furnace Installation | $3,500–$8,000+ | 1–2 days |
Tip: Many furnaces carry longer warranties on heat exchangers and shorter on electronics. Labor is often not covered beyond the first year unless an extended plan was purchased.
Stay Warm And Protect Your Home While Waiting
If a furnace stopped working and service is scheduled, take steps to maintain comfort and prevent damage without creating new hazards.
Use UL-listed space heaters with tip-over protection. Keep them three feet from anything combustible. Plug directly into wall outlets, not power strips. Do not leave them unattended or running overnight.
Prevent frozen pipes by opening sink cabinets, especially on exterior walls, and letting faucets drip slightly. Keep interior doors open for air circulation.
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Layer clothing, use extra blankets, and close off seldom-used rooms to concentrate heat. Use heavy curtains at night and open them during sunny hours for passive warming.
Do not use ovens, stovetops, or grills for space heating. These create fire and CO risks. Fireplaces should be used with the damper open and proper ventilation.
Preventive Maintenance To Avoid The Next No-Heat Call
Simple routine care reduces breakdowns and improves efficiency. Most tasks are homeowner-friendly and take minutes.
Change Filters Regularly: Check 1-inch filters monthly and replace at least every 60–90 days. For thicker media (3–5 inch), inspect quarterly and replace per manufacturer guidance. Choose a MERV rating that balances filtration and airflow.
Keep Vents Clear: Before winter storms, mark and check PVC intake/exhaust terminations. Clear snowdrifts promptly. Indoors, keep returns and supplies unblocked.
Annual Professional Service: A fall tune-up typically includes combustion checks, electrical testing, cleaning burners, verifying gas pressures, and ensuring condensate systems drain properly. Request a written report.
Thermostat Optimization: For programmable or smart thermostats, set reasonable setbacks (3–5°F). Extreme setbacks can cause long recovery times and comfort issues in very cold weather.
Carbon Monoxide Detectors: Test monthly and replace batteries semiannually. Replace detectors every 5–7 years, per manufacturer instructions.
Duct Sealing And Insulation: Sealing accessible ducts with mastic and insulating attic or crawlspace runs can improve delivery temperatures and reduce short cycling.
Gas, Electric, And Oil Furnace Differences
Most U.S. homes use natural gas furnaces. Electric furnaces and oil furnaces have different failure patterns and checks.
Gas Furnaces: Common issues include ignition faults, pressure switch problems, limit trips from airflow restrictions, and condensate blockages in high-efficiency models.
Electric Furnaces: Failures often involve sequencers, heating elements, or breakers. If the blower runs but air is not warm, an element or high-limit switch may have opened. Service is recommended due to high voltage.
Oil Furnaces: No-heat issues can involve nozzle clogs, dirty filters, or burner ignition problems. Reset the burner only once. If it trips again, call a qualified oil technician.
Cold-Weather Startup Sequence: What You Should See
Knowing the normal sequence helps pinpoint where a “furnace not turning on” condition occurs.
1) Thermostat calls for heat. 2) Inducer motor starts to clear the heat exchanger. 3) Pressure switch proves draft. 4) Ignition sequence begins (hot surface igniter glows or spark starts). 5) Gas valve opens; burners light. 6) Flame sensor confirms flame. 7) Blower starts after a short delay.
If the sequence stops at a step, the associated component or safety is suspect. For example, no inducer may point to a power or control issue. Ignition without flame sense indicates a sensor or grounding issue.
Common Questions About A Furnace Stopped Working
Why does the furnace run for a minute and shut off? That pattern often points to flame-sensing problems or overheating from poor airflow. Replace the filter, open vents, and check intake/exhaust. If it persists, service is needed.
Is it safe to clean a flame sensor? Light oxidation is common, but cleaning requires care to avoid damage. Because the sensor confirms safe combustion, it is best handled during a professional tune-up.
How often should filters be changed? In most homes, check monthly and change every 1–3 months for 1-inch filters. Homes with pets, renovations, or allergies may need more frequent changes.
Why does the breaker keep tripping? Repeated trips indicate electrical faults, seized motors, or short circuits. Do not keep resetting. Call a professional to prevent wiring damage or fire risk.
What temperature should be expected at vents? Gas furnaces typically deliver supply air 20–40°F warmer than the return air. Extremely low rise suggests burner or element issues; very high rise can indicate restricted airflow.
Can snow really stop a furnace? Yes. High-efficiency furnaces draw and exhaust through outdoor PVC pipes. Snow or ice can block them, causing pressure switch faults and shutdowns. Keep those terminations clear.
Will a smart thermostat help? Smart thermostats can improve comfort and scheduling, but they do not fix mechanical issues. Ensure compatibility with the furnace and C-wire availability for reliable operation.
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At-A-Glance Quick Checklist
- Safety: No gas smell, CO alarms working, clear area around furnace.
- Thermostat: Heat mode, setpoint up, fresh batteries, schedules checked.
- Power: Furnace switch On, breaker reset once, door firmly closed.
- Airflow: New filter, open vents and returns, clear furnace area.
- Gas/Ignition: Gas valve open, one safe reset only, observe startup.
- Condensate/Vents: Clear drain, pump operating, outdoor PVC pipes unobstructed.
- Call Pro If: Persistent error codes, breaker trips, ignition fails, or any safety concerns.
Following these steps can restore heat quickly in many cases. When issues persist, prompt professional service protects safety, prevents costly damage, and ensures reliable comfort through the coldest days.
How to Get the Best HVAC Prices
- Firstly, keep in mind that installation quality is always the most important thing for residential HVAC project. So never sacrifice contractor quality for a lower price.
- Secondly, remember to look up the latest rebates as we talked above.
- Thirdly, ask for at least 3 bids before you make the decision. You can click here to get 3 free estimates from your local contractors, and this estimate already takes rebates and tax credit into consideration and filter unqualified contractors automatically.
Lastly, once you chose the right contractor, remember to use the tactics from this guide: Homeowners Tactics When Negotiating with HVAC Dealer to get the final best price.
