What to Do if Your Furnace Stops Working

The following guide explains practical troubleshooting steps and safety precautions when a furnace stops working, helping homeowners quickly identify problems and decide whether a repair or professional service is needed.

Quick Steps	Action
1	Check thermostat and settings
2	Verify power and breakers
3	Inspect pilot light/ignition
4	Replace filter and check vents
5	Look for error codes or lights

How To Use This Guide

This article walks through step-by-step troubleshooting for when a furnace stops working, covering simple fixes homeowners can safely do and signs that require calling an HVAC technician.

Initial Safety Precautions

Safety first: If there is a smell of gas, a hissing sound, visible flames where they shouldn’t be, or any sign of carbon monoxide, evacuate the building immediately and call 911 and the gas company. Do not attempt to restart the furnace in the presence of a gas leak.

Before opening access panels or performing any inspection, turn off power to the furnace at the thermostat and the circuit breaker to avoid electrical shock.

Check Thermostat Settings And Batteries

A common reason a furnace stops working is incorrect thermostat settings or dead batteries. Confirm the thermostat is set to heat and the temperature setpoint is above current room temperature.

For battery-powered or programmable thermostats, replace batteries and verify the display is functioning. If the thermostat is blank or unresponsive, replace batteries or test with another thermostat if available.

Verify Power Supply And Circuit Breaker

If the furnace does not power up, confirm the home’s circuit breaker for the furnace has not tripped. Turn the breaker fully off and back on to reset it if necessary.

Check the furnace switch (usually a light switch on or near the unit) and the emergency shutoff switch near the furnace or on the gas line. Ensure power cords are plugged in and any external switches are on.

Inspect Pilot Light Or Electronic Ignition

Older furnaces use a pilot light; modern furnaces use an electronic ignition system. For pilot light furnaces, confirm the pilot is lit and producing a steady blue flame. If it is out, follow the manufacturer’s relighting procedure or call a pro if unsure.

For furnaces with electronic ignition, listen for the ignition sequence when the thermostat calls for heat. If there is clicking without ignition, it may indicate a faulty igniter, gas valve issue, or insufficient gas supply.

Confirm Gas Supply And Shutoff Valve

Ensure the gas valve to the furnace is open by checking the valve handle is parallel to the gas pipe. If other gas appliances are working but the furnace is not, suspect internal furnace components.

If the furnace smells like gas or a gas leak is suspected, leave immediately, avoid using electrical devices or phones indoors, and call emergency services and the gas utility from a safe location.

Replace Or Check The Air Filter

A clogged air filter can cause a furnace to stop working or short-cycle because of restricted airflow. Inspect the filter and replace it if dirty. Many homes benefit from replacing filters every 1–3 months during heavy use.

Tip: Use the correct filter size and MERV rating recommended by the furnace manufacturer to avoid strain on the blower motor.

Inspect Vents And Registers

Blocked supply vents and return grills can reduce airflow and force the furnace to shut down on high-limit safety switches. Check that all registers are open and unobstructed.

Ensure the return vent is clear and that furniture or rugs are not blocking airflow. Clean dust and debris from registers and consider vacuuming the return grate.

Check The Condensate Drain (High-Efficiency Furnaces)

High-efficiency condensing furnaces produce condensate that drains away. A clogged condensate drain or safety switch can shut the furnace down. Inspect the small PVC drain line and trap for blockages or algae.

Clear the drain with a wet/dry vacuum or a bleach/water solution if safe to do so. Reset the condensate overflow switch if accessible and the drain has been cleared.

Look For Error Codes Or Indicator Lights

Many furnaces have a diagnostic LED or control board display that flashes error codes. Check the furnace access panel or the owner’s manual to decode the LED flash pattern.

Common codes point to issues such as ignition failures, flame rollout, flame sensor faults, or blower motor problems. Recording the code sequence before resetting the unit helps HVAC technicians diagnose the problem.

Inspect Flame Sensor And Burners

A dirty or corroded flame sensor often causes intermittent shutdowns. This thin metal rod detects the flame; it can be carefully cleaned with fine emery cloth or steel wool, then reinstalled.

Visually inspect burners for rust, debris, or uneven flames. Yellow or flickering flames may indicate dirty burners or improper combustion and warrant professional cleaning and tuning.

Examine The Blower Motor And Fan

If the furnace produces heat but does not distribute it, the blower motor or fan may be failing. Listen for unusual noises, grinding, or squealing which indicate worn bearings or failing motors.

Check the blower capacitor and motor connections if comfortable doing so, but avoid electrical work without proper training. A failed blower often requires replacement by a licensed technician.

Check The Limit Switch And Safety Controls

The high-limit switch shuts the furnace down if temperatures get too high, typically caused by restricted airflow or a malfunctioning blower. Resetting the limit switch is sometimes possible, but resolving the underlying airflow problem is essential.

Other safety controls include rollout switches and pressure switches. If a safety switch has tripped repeatedly, schedule a professional inspection to diagnose the cause.

Basic Reset Procedures

Sometimes a simple reset restores normal function. Turn off the furnace at the thermostat and power switch, wait 60 seconds, then turn power back on. Allow the furnace several minutes to complete its startup sequence.

A reset may clear minor errors, but repeated resets suggest a recurring fault that requires service. Do not repeatedly reset a tripping breaker or safety switch without addressing the root cause.

When To Call An HVAC Professional

Call a licensed HVAC technician if troubleshooting does not restore operation, or if any of the following occur: persistent gas odors, repeated tripping of safety switches, no pilot or ignition after attempts, electrical burning smells, or intermittent operation.

Professional service is recommended for complex issues such as gas valve replacement, major electrical repairs, heat exchanger inspection, or if carbon monoxide detectors indicate a problem.

Estimated Repair Costs And What To Expect

Minor repairs such as thermostat replacement or cleaning a flame sensor often cost $75–$300, while blower motor replacements, heat exchanger repairs, or major component replacements can range from $500 to $3,000 depending on labor and parts.

Seasonal peak demand may increase wait times and prices. Obtain multiple estimates, verify contractor licensing and insurance, and request written estimates detailing parts, labor, and warranties.

Preventive Maintenance To Avoid Future Breakdowns

Seasonal preventive maintenance reduces the chance a furnace will stop working unexpectedly. Recommended tasks include filter replacement every 1–3 months, annual professional tune-ups, cleaning burners and flame sensors, and checking venting and flue integrity.

Scheduling maintenance before the heating season and enrolling in a service plan can improve reliability and efficiency, often saving money over time.

Carbon Monoxide Safety And Detectors

Because furnaces can produce carbon monoxide (CO), ensure working CO detectors are installed on each floor and near sleeping areas. Replace detectors per manufacturer guidance and test them monthly.

If a CO alarm sounds, evacuate immediately and call emergency services. Never ignore an alarm or a persistent unexplained headache, dizziness, or nausea while the furnace runs.

DIY Troubleshooting Checklist

• Confirm thermostat settings and replace batteries
• Check circuit breaker and power switch
• Inspect pilot light or listen for ignition
• Replace dirty air filter
• Clear vents and returns
• Inspect condensate drain for blockages
• Record and lookup any diagnostic LED codes
• Reset the furnace once after addressing visible issues

Choosing A Qualified HVAC Contractor

When professional help is needed, choose a contractor who is licensed, insured, and has positive reviews. Ask about experience with the specific furnace brand and model.

Request written estimates, ask about warranties on parts and labor, and confirm the contractor will perform a full system safety inspection including carbon monoxide and combustion analysis when applicable.

Energy Efficiency And Replacement Considerations

If a furnace is old and frequently breaks down, consider replacement. Modern high-efficiency furnaces can lower energy bills and reduce repair frequency. Compare AFUE ratings, size the new system correctly, and consider a combined heating and cooling solution for overall efficiency.

Factor in long-term energy savings, rebate programs, and potential cost reductions from improved insulation or thermostat upgrades when deciding between repair and replacement.

Additional Resources And Documentation

Keep the furnace owner’s manual, maintenance records, and any service invoices in one place. Manuals often include troubleshooting steps, error code charts, and manufacturer contact information.

Find local licensed HVAC professionals through trade associations, utility company lists, or Better Business Bureau searches to ensure reliable service.

Key Takeaways And Next Steps

Key points: Start with safety and basic checks (thermostat, power, filter, vents), inspect ignition and gas supply, consult error codes, and avoid repeated resets. Call a licensed HVAC technician for gas, electrical, or complex mechanical issues.

Document symptoms, times, and any error codes to share with the technician to speed diagnosis and repair. Regular maintenance is the best defense against unexpected furnace failures.