Furnace Won’t Stay Lit Troubleshooting and Fixes

The furnace won’t stay lit is a common homeowner complaint that can leave homes cold and utility bills high. This article explains the most likely causes, safe troubleshooting steps, and when to call a professional. It focuses on practical, U.S.-centric guidance and clear cost and safety context to help readers diagnose and resolve the problem responsibly.

Symptom	Likely Cause	Possible Fix
Pilot Light Ignites Then Goes Out	Dirty thermocouple or weak pilot flame	Clean/adjust thermocouple or replace thermocouple; call pro for gas valve issues
Burners Ignite But Flame Dies	Faulty flame sensor or dirty burners	Clean or replace flame sensor; clean burners
Igniter Clicks But No Flame	Gas valve or ignition control failure	Test gas supply; replace gas valve or control board (professional)
Furnace Starts Then Shuts Off Shortly After	Limit switch, pressure switch, or clogged filter	Replace filter, test switches, clear venting; call tech if persistent

How A Typical Gas Furnace Ignition System Works

Understanding why a furnace won’t stay lit begins with the ignition sequence. Modern gas furnaces use an electronic igniter or a standing pilot with a thermocouple. The thermostat calls for heat, the ignition system lights the burner, a flame sensor confirms ignition, and the blower runs after the heat exchanger reaches temperature. Safety switches (limit, pressure) and gas valves control operation and shut the system down if a problem appears.

Common Causes When A Furnace Won’t Stay Lit

Identifying the correct cause reduces repair time and cost. The most frequent issues include problems with the flame sensor, thermocouple or pilot assembly, gas supply or gas valve, dirty burners, safety switches, and venting problems that trigger safety shutdowns.

Flame Sensor Problems

The flame sensor is a thin metal rod that detects a flame and signals the control board to keep the gas valve open. A dirty or corroded sensor can fail to sense the flame, causing the furnace to shut off after a few seconds. Cleaning often restores function; replacement costs are moderate.

Thermocouple Or Pilot Issues

Older furnaces use a pilot light and a thermocouple. The thermocouple senses pilot flame heat to allow the main gas valve to open. If the thermocouple is weak, misaligned, or dirty, it can cool and close the gas valve, so the pilot won’t stay lit.

Gas Supply Or Gas Valve Failure

If the gas supply is interrupted or the gas valve or control board malfunctions, the burners may not stay lit. Gas valve problems require a certified technician because they involve fuel supply and safety controls.

Safety Switches And Venting Problems

Limit switches, pressure switches, and blocked vents can cause shutdowns. A clogged air filter or blocked exhaust vent can trigger the high-limit switch, stopping the burner until conditions normalize. Pressure switches or inducer motor faults also interrupt ignition sequences.

Dirty Burners And Ignition Components

Accumulated dirt, rust, or spider webs can obstruct pilot flames, burner ports, or the sensor’s line of sight. Regular cleaning of burners and ignition components prevents intermittent flame failures and improves efficiency.

Step-By-Step Troubleshooting When The Furnace Won’t Stay Lit

Before performing checks, turn off power to the furnace and shut off the gas if any smell of gas is present. If there is a strong odor, evacuate and call the gas company or 911. For all other steps, if any uncertainty exists, contact a licensed HVAC technician.

1. Check Thermostat And Power

Confirm the thermostat is set to heat and the temperature setpoint is above room temperature. Verify the furnace switch and circuit breaker are on. A power interruption can reset ignition controls and create intermittent issues.

2. Inspect And Replace Air Filter

A clogged filter can cause short-cycling and high-limit trips. Replace the filter if dirty. This is a simple maintenance task that often resolves running-too-short problems.

3. Observe The Ignition Sequence

With the furnace powered, set a call for heat. Watch whether the pilot/igniter comes on, the burners light, and how long they remain lit. Note error codes or diagnostic LED blink sequences on the control board and refer to the furnace manual.

4. Clean The Flame Sensor

If the burners light then the furnace shuts off, the flame sensor likely needs cleaning. Turn off power, remove the sensor, gently clean it with fine-grit sandpaper or steel wool, then reinstall. This often fixes a furnace that won’t stay lit within moments.

5. Check Pilot Light And Thermocouple (If Applicable)

For standing pilot systems, ensure the pilot flame envelops the thermocouple. Clean the pilot assembly and adjust the flame if it’s weak. Replace the thermocouple if cleaning and adjustment don’t help. Pilot relighting should follow the manufacturer’s instructions and local codes.

6. Inspect Burners And Orifices

Turn off gas and power, then inspect burners for rust, dirt, or spider webs. Clean with a soft brush and compressed air. Misaligned or partially blocked burners can cause flickering or extinguishing flames.

7. Evaluate Venting And Inducer Motor

Ensure exhaust flue and intake vents are clear of debris, nests, or snow. A blocked vent causes pressure switch trips and stops ignition. Listen for unusual inducer motor noise that indicates a failing draft system.

8. Check Pressure And Limit Switches

Inspect pressure switch tubing for blockages and test the high-limit switch for continuity if the furnace cools and then won’t relight. Replace any faulty switch. These components are safety-critical and sometimes require professional testing.

9. Verify Gas Supply And Gas Valve Operation

Confirm other gas appliances are working to rule out a supply issue. If gas reaches the furnace but burners won’t stay lit, the gas valve or control board may be failing. Gas valve replacement should be performed by a licensed technician due to safety and code requirements.

When To Stop Troubleshooting And Call A Professional

The homeowner should stop and call a licensed HVAC technician if there is a gas smell, if repairs require gas valve replacement, if electrical components look damaged, or if the problem persists after basic cleaning and filter replacement. Technicians can safely test gas pressure, control boards, and sealed combustion systems.

Safety Considerations When A Furnace Won’t Stay Lit

Safety is paramount. If a strong gas odor is present, leave the home, call the gas company and emergency services. Never attempt complex gas system repairs without certification. Use a CO detector and ensure it is operational because faulty combustion can produce carbon monoxide.

Estimated Costs And Typical Replacements

Costs vary by region and furnace type. Typical ranges for common repairs: Flame sensor replacement: $80–$250; Thermocouple replacement: $50–$150; Pilot assembly cleaning/adjustment: $60–$200; Gas valve replacement: $300–$900; Control board replacement: $200–$800; Inducer motor: $200–$600. Always get multiple quotes from licensed HVAC companies.

Maintenance Tips To Prevent A Furnace From Not Staying Lit

• Replace Filters Every 1–3 months depending on usage and filter type.
• Schedule Annual Tuneups Professional inspection and cleaning before the heating season improves reliability and safety.
• Keep Vents Clear Maintain intake and exhaust clearance outside the home and remove debris.
• Clean Burners And Ignition Components During annual service, have burners and sensors cleaned to prevent flame sensing failures.
• Install And Test CO Detectors Place detectors near sleeping areas and test them monthly.

Diagnostic Checklist For Technicians And Savvy Homeowners

1. Confirm thermostat operation and power to the furnace.
2. Replace or inspect air filter.
3. Observe ignition sequence and note any control board codes.
4. Clean flame sensor and pilot assembly if present.
5. Inspect burners for blockage and clean.
6. Check venting, inducer motor, and pressure switch tubing.
7. Test limit switch and other safety devices for proper operation.
8. Verify gas supply pressure and gas valve operation (technician).
9. Document repairs and recommend preventive service plan.

Frequently Asked Questions About A Furnace That Won’t Stay Lit

How Long Should A Furnace Flame Stay Lit?

Once lit, the flame should remain steady for the full heating cycle until the thermostat is satisfied. If the flame extinguishes within seconds or a few minutes, a sensor, safety switch, or gas issue is likely.

Is It Safe To Clean The Flame Sensor Yourself?

Yes, cleaning a flame sensor is a common DIY task if the homeowner is comfortable turning power off, accessing the component, and using fine-grit sandpaper. If unsure, a service call is advisable.

Can A Dirty Filter Cause The Furnace To Shut Off?

Yes, a heavily clogged filter can cause overheating and trigger the high-limit switch, resulting in short cycling and furnace shutdowns.

Will A New Thermocouple Fix A Pilot That Won’t Stay Lit?

Often, yes. A weak thermocouple commonly causes pilot flame loss. If the pilot flame itself is weak due to blockage, cleaning may be required as well.

Resources And Next Steps

When a furnace won’t stay lit, begin with simple checks: power, thermostat, filter, and flame sensor. Use professional help for gas valve, control, or combustion-related repairs. Maintain annual service to reduce repeat issues and prioritize safety by using CO detectors and responding to gas odors immediately.