Furnace Flame Won’t Stay Lit: Causes, Troubleshooting, and Fixes

The furnace flame not staying lit is a common issue that causes intermittent heat, safety shutdowns, and costly energy waste. This article explains the most likely causes, step-by-step troubleshooting, safety checks, and repair options so homeowners can identify problems and decide whether to DIY or call a professional. Key focus: pilot and ignition systems, flame sensors, gas supply, airflow and safety switches.

Symptom	Possible Cause	Typical Fix
Pilot lights then goes out	Faulty thermocouple/pilot assembly	Clean/replace thermocouple or pilot assembly
Igniter sparks but flame dies	Dirty flame sensor	Clean or replace flame sensor
Burners ignite then shut off	Ventilation/safety switch or gas pressure issues	Inspect venting, check pressure, call tech

How Furnaces Light And Why Flames Go Out

Understanding how the furnace ignition and safety systems work helps pinpoint why the flame won’t stay lit. Modern furnaces use either a standing pilot with a thermocouple or an electronic ignition system with a hot surface igniter or direct spark.

Once ignition occurs, the furnace uses a flame sensor to confirm combustion and multiple safety switches (pressure switch, limit switch, rollout switch) to monitor proper operation. If the sensor or switches do not detect a safe flame or conditions, the control board will shut the gas off, causing the flame to go out.

Common Causes And Diagnostic Checks

Poor Pilot Or Thermocouple Function (Standing Pilot Furnaces)

In older furnaces with a standing pilot, the thermocouple detects the pilot flame. A weak or misaligned pilot flame or a failing thermocouple will let the main gas valve close. Signs include a pilot that lights but won’t stay lit or needs frequent relighting.

Basic checks: verify pilot flame size and position, inspect the thermocouple tip for soot or damage, and test continuity if equipped with a multimeter. Replacing a thermocouple is inexpensive and often resolves the issue.

Dirty Or Faulty Flame Sensor (Electronic Ignition Furnaces)

Electronic furnaces often use a flame sensor rod that senses ionized combustion. A thin coating of soot or oxidation can prevent detection, causing the control board to cut gas after ignition.

Cleaning the flame sensor with fine grit sandpaper or steel wool and then reinstalling it often restores reliable operation. If cleaning doesn’t help, replacement is typically required.

Igniter Problems: Hot Surface Igniter Or Spark Igniter

If the igniter lights the burners but the flame dies immediately, inspect the igniter and control sequence. A weak hot surface igniter can fail to sustain combustion, and a failing spark igniter can produce intermittent sparks.

Replacement of a cracked or faltering hot surface igniter is common. A visual inspection and a multimeter resistance test can indicate failure.

Airflow Or Combustion Air Problems

Restricted airflow or inadequate combustion air can cause flame instability. Clogged filters, blocked return vents, closed supply registers, or restrictive ductwork reduce combustion airflow and cause irregular flame or safety shutdowns.

Check and replace filters, open blocked vents, and verify airflow at registers. Ensure the furnace room has adequate ventilation if the unit draws air from indoors.

Blocked Or Improper Venting

Exhaust vent restrictions or a blocked flue cause backpressure and incomplete combustion, triggering safety switches that shut the gas off. Symptoms include burners igniting then turning off, or error codes related to vent safety.

Inspect outside vent terminals for nests, debris, snow, or ice. Ensure proper slope and integrity of vent pipes. For power-vent furnaces, confirm the inducer motor runs and the pressure switch closes.

Faulty Pressure Switch Or Draft Proving Switch

The pressure switch verifies the inducer fan is moving combustion gases through the vent. If the switch does not close, the furnace will shut off the gas. Common causes include blocked venting, failed inducer motor, or failing switch.

Listen for the inducer motor startup. If it runs but the furnace still shuts down, the pressure switch or tubing may be clogged. A technician can test switch continuity and inducer performance.

Gas Supply And Regulator Issues

Insufficient gas pressure from the utility or a failing internal gas valve can cause flames to die shortly after ignition. Intermittent gas flow or low pressure will make the burners struggle to stay lit.

Confirm other gas appliances work. If multiple appliances show low flame, contact the gas utility. If isolated to the furnace, a professional should test manifold pressure and inspect the gas valve.

Rollout Switch Or Limit Switch Activations

Rollout switches detect flames escaping the combustion chamber and will cut fuel if triggered. A tripped rollout switch or high-limit switch indicates overheating, blocked heat exchanger, or cracked exchanger.

These are serious safety concerns. If rollout or limit switches are involved, avoid repeatedly resetting them and call an HVAC professional for inspection and repair.

Step-By-Step Homeowner Troubleshooting

Follow safety-first steps before attempting any fixes. If gas smell, heavy corrosion, or structural issues are present, contact a professional immediately.

1. Turn off power and gas to the furnace before inspecting internal components. Check for gas odor; if present, evacuate and call the utility.
2. Replace the furnace filter and ensure supply/return vents are open to rule out airflow restrictions.
3. If the furnace has a standing pilot, observe the pilot flame. It should envelop the thermocouple tip; if weak, clean or replace the pilot assembly or thermocouple.
4. For electronic ignition, open the access panel, visually inspect the flame sensor for buildup, remove it, and clean gently with fine sandpaper or emery cloth, then reinstall.
5. Inspect the vent termination outside for blockages and ensure the inducer motor runs during the ignition cycle. Listen for unusual sounds indicating a failing inducer.
6. Check error codes on the furnace control board or LED diagnostic display and consult the manual for meaning.

When To Call A Professional

Contact a licensed HVAC technician when the issue involves gas pressure, suspected cracked heat exchanger, persistent flame loss after cleaning, electrical component failure, or safety switch trips. Problems involving the gas valve, internal pressure testing, or combustion analysis require trained professionals and specialized tools.

Immediate professional service is required if there is a gas smell, flame rollout is observed, or the furnace repeatedly trips safety devices.

Repair And Replacement Costs

Costs vary by region and part. Typical ranges:

• Thermocouple replacement: $50–$150 (parts and labor).
• Flame sensor cleaning: $0–$75 if DIY; professional cleaning $75–$150.
• Hot surface igniter replacement: $100–$300 installed.
• Gas valve replacement: $300–$800 installed.
• Inducer motor or pressure switch: $200–$500 installed.

Labor rates and part prices vary, so obtain multiple estimates for major repairs or replacements.

Preventive Maintenance To Keep The Flame Reliable

Regular maintenance reduces the risk of flame issues and improves safety and efficiency. Recommended practices include annual professional tune-ups before heating season, filter replacement every 1–3 months, and keeping vent terminations clear.

During maintenance, a technician will inspect the ignition system, clean the flame sensor and burners, test gas pressure, check safety switches, and perform combustion analysis if needed.

Safety Tips And Best Practices

• Never attempt gas valve or internal combustion repairs without proper training.
• If a persistent gas odor is present, shut off the gas supply at the meter and leave the building, then call the gas company.
• Do not repeatedly bypass safety switches; they protect against dangerous conditions.
• Keep carbon monoxide detectors in the home and test them regularly.

FAQ

Why Does The Furnace Light Then Go Out After A Few Seconds?

That’s usually a detection or safety timeout problem. The control board permits ignition, but if the flame sensor or safety switches do not confirm proper combustion, the system shuts the gas off.

Can A Dirty Flame Sensor Cause The Flame To Go Out?

Yes. A dirty or corroded flame sensor is one of the most common causes of burners igniting and then extinguishing. Cleaning often restores normal operation.

Is It Dangerous If The Furnace Flame Keeps Going Out?

Intermittent flames can indicate unsafe combustion or gas flow issues. While the system is designed to shut down unsafe conditions, repeated problems should be inspected promptly to avoid carbon monoxide risks or other hazards.

Can Homeowners Replace A Flame Sensor Or Thermocouple?

Yes, homeowners with basic mechanical skills and the power turned off can replace sensors and thermocouples. Follow manufacturer instructions and shut off power and gas before working on the furnace.

Key Indicators To Mention To A Technician

When calling for service, provide these details: furnace make and model, specific symptoms (pilot lights vs. electronic ignition, error code numbers), timing (always, intermittent, after startup), any recent service or weather events, and whether other gas appliances are affected.

Clear, specific information helps technicians diagnose problems faster and prepare the right parts.

Useful Parts And Tools For DIY Troubleshooting

Item	Purpose
Fine sandpaper/emery cloth	Clean flame sensors
Multimeter	Check continuity and voltages
Replacement thermocouple or flame sensor	Common replacement parts
Shop vacuum or compressed air	Remove dust from burners and venting

Always follow furnace manufacturer instructions and local codes when performing maintenance or replacing parts.

Bottom Line: A flame that won’t stay lit most often stems from a dirty flame sensor, pilot/thermocouple issues, ignition failure, venting problems, or gas pressure/safety switch faults. Start with safe, simple checks like filters and sensor cleaning, but enlist a licensed HVAC technician for gas-related or safety-critical repairs.