The problem where a furnace flame starts then goes out is a common heating issue that can leave homeowners without heat and with safety concerns. This article explains the typical causes, diagnostic steps, and repair options for furnaces that ignite but fail to sustain a flame, helping readers understand when to troubleshoot and when to call a professional.
| Symptom | Likely Cause | Possible Fix |
|---|---|---|
| Flame lights then extinguishes within seconds | Faulty flame sensor or dirty sensor | Clean or replace flame sensor |
| Ignition clicks but no sustained flame | Pilot assembly or intermittent gas valve | Inspect pilot/replace valve or control board |
| Flame cycles on and off repeatedly | Blocked vent or pressure switch issue | Clear vent/repair or replace pressure switch |
How Furnaces Ignite And Why Flames Must Stay Lit
Most modern gas furnaces use an electronic ignition system and a flame sensor or thermocouple to confirm combustion. The furnace control board calls for heat, the ignitor lights the gas, and the flame sensor signals the control board that the burner is burning. If the sensor does not detect a stable flame, the control board shuts the gas off to prevent unburned fuel buildup.
Understanding this safety loop is essential because repeated shutdowns can damage components and create safety risks like gas buildup or carbon monoxide if components are compromised.
Common Causes When A Furnace Flame Starts Then Goes Out
Dirty Or Faulty Flame Sensor
The flame sensor is a thin metal rod placed in the burner flame to confirm ignition. Carbon buildup or corrosion on the sensor prevents it from detecting the flame, causing the system to shut off the gas after ignition. Cleaning is often effective; replacement is needed if the sensor is corroded or cracked.
Malfunctioning Ignitor
Ignitors can crack or degrade over time. A weak or intermittent ignitor may start the burner but fail to sustain a stable flame, which can lead to shutdowns. Visible cracks or inconsistent glowing indicate replacement is required.
Pilot Assembly Problems (Older Furnaces)
Older furnaces with a standing pilot light can have issues where the pilot is weak or the thermocouple fails. A weak pilot flame may not keep the thermocouple hot enough to signal the gas valve, causing the main burner to light briefly then go out.
Pressure Switch Or Venting Blockages
High-efficiency furnaces rely on a draft inducer motor and pressure switch to confirm proper venting. If the pressure switch senses poor draft or a blocked vent, it may interrupt the ignition sequence after the flame lights. Debris, bird nests, or snow can obstruct vents and cause this issue.
Faulty Gas Valve Or Gas Supply Issues
Intermittent gas pressure or a failing gas valve can lead to the flame starting and then extinguishing. Restricted supply lines, partially closed shutoff valves, or failing internal gas valve components can cause unstable burner operation.
Control Board Or Wiring Problems
The furnace control board manages ignition, flame sensing, and safety interlocks. Faulty relays, intermittent wiring connections, or a failing board can cut power during the ignition cycle, causing the flame to go out. Electrical checks can reveal loose wires or damaged connectors.
How To Diagnose A Furnace That Starts Then Goes Out
Safety First
Before any inspection, turn off power and gas to the furnace if accessible, and follow manufacturer safety instructions. If a homeowner smells gas or detects carbon monoxide, evacuate and call emergency services and the gas company immediately.
Observe The Sequence
Run the thermostat through a heat cycle and observe. Note whether the furnace completes the full ignition sequence, whether the ignitor glows consistently, whether the burners light evenly, and the time between ignition and shutdown. These observations narrow down likely causes.
Check The Flame Sensor
Locate the flame sensor (a rod near the burners). Remove and inspect for dark carbon buildup or corrosion. Cleaning typically involves gently sanding the sensor rod with fine-grit sandpaper or a Scotch-Brite pad and wiping with a cloth. Reinstall and test. If problems persist, replace the sensor.
Inspect Ignitor Condition
Look for cracks or visible wear on the ignitor. If the ignitor flickers, fails to glow, or shows physical damage, replace it. Note that handling ceramic ignitors requires care because oils from hands reduce lifespan.
Examine Venting And Inducer System
Check exterior vent terminations for obstructions and ensure the inducer motor runs properly during ignition. Test the pressure switch with a multimeter if comfortable; it should close when the inducer creates draft. A stuck-open or stuck-closed pressure switch may require replacement.
Test Gas Valve And Supply
Confirm the gas shutoff is fully open and there are no kinks in gas piping where applicable. Measuring gas valve operation requires a qualified technician and specialized tools because of safety risks. Suspected gas valve issues should prompt a service call.
Inspect Control Board And Wiring
Look for burned components, loose connectors, or corroded terminals on the control board. Wiggle wiring gently to check for intermittent connections. Digital error codes on the control board LED or thermostat error messages can guide diagnosis.
Troubleshooting Steps Homeowners Can Try
- Power Cycle The Furnace: Turn off power at the breaker, wait 60 seconds, and restore power to clear minor control faults.
- Replace The Air Filter: A severely clogged filter can restrict airflow and trigger safety shutdowns that affect combustion.
- Clean The Flame Sensor: Remove, sand lightly, and reinstall the sensor. This is a common, low-cost fix.
- Clear External Vents: Remove debris, nests, or snow from the exhaust and intake vents to restore proper draft.
- Reset Thermostat Settings: Ensure the thermostat is set to heat and on an appropriate mode; try bypassing the programmable schedule for testing.
When To Call A Professional HVAC Technician
Certain issues require licensed technicians, particularly any that involve gas components, combustion analysis, or internal pressure testing. Call a technician if the furnace repeatedly fails to sustain a flame after basic cleaning, if there is a gas odor, if the pilot assembly or gas valve appears defective, or if electrical diagnostics point to a failing control board.
Technician-level services include gas pressure measurement, combustion efficiency testing, replacement of gas valves or control boards, and in-depth leak or carbon monoxide testing.
Common Repairs And Estimated Costs
| Repair | Typical Cost Range (U.S.) |
|---|---|
| Clean Or Replace Flame Sensor | $75–$200 |
| Replace Ignitor | $150–$400 |
| Pressure Switch Replacement | $150–$350 |
| Gas Valve Replacement | $300–$900 |
| Control Board Replacement | $200–$800 |
Prices vary by region, furnace model, and whether additional labor or parts are required. Obtaining multiple estimates and choosing technicians with good reviews and proper licensing is recommended.
Preventive Maintenance To Avoid Flame Issues
Regular maintenance reduces the chance of a furnace flame starting then going out. Annual inspections by a professional ensure component integrity, proper gas pressure, and safe venting. Seasonal HVAC tune-ups commonly include cleaning the burners, checking the flame sensor and ignitor, inspecting heat exchangers, and testing safety controls.
Homeowner maintenance includes changing filters every 1–3 months, keeping vents and registers clear, and ensuring outdoor vent terminations are free of obstructions.
Safety Considerations And Carbon Monoxide Risks
Repeated ignition failures can increase the risk of partial combustion and carbon monoxide production if the heat exchanger or venting is compromised. Install and maintain carbon monoxide detectors near sleeping areas and the furnace. If the furnace emits a strong burning odor, excessive soot, or a yellow burner flame instead of blue, shut it down and call a professional.
FAQs About Furnace Flames That Start Then Go Out
Why Does The Flame Light Blue Then Turn Yellow?
A healthy gas flame should be mostly blue. A yellow or flickering flame can indicate incomplete combustion, dirty burners, or air-gas mixture problems, and should be inspected promptly.
Is It Safe To Keep Restarting The Furnace When The Flame Goes Out?
Repeated restarts can stress components and potentially create unsafe conditions. If the furnace does not operate normally after one or two tries, stop attempting restarts and contact a professional.
Can A Dirty Air Filter Cause The Flame To Go Out?
A severely clogged air filter can restrict airflow, trigger high-limit switches, and alter combustion dynamics, which may indirectly contribute to ignition failures. Regular filter changes are an important preventive measure.
Choosing The Right HVAC Technician
Select a licensed, insured HVAC contractor with experience in gas furnace diagnostics and repairs. Ask for references, verify license numbers, and request written estimates. Confirm the technician performs safety checks like carbon monoxide testing and combustion efficiency verification after repairs.
Warranty considerations are important: ensure replacement parts and labor by a contractor do not void existing manufacturer warranties.
Key Takeaways
When a furnace flame starts then goes out, likely causes include a dirty flame sensor, failing ignitor, venting or pressure switch issues, gas valve problems, or control board/wiring faults. Homeowners can safely perform simple checks like cleaning the flame sensor, clearing vents, and replacing filters. For any gas-system work, persistent failures, or safety concerns, a licensed HVAC professional should be contacted.
Proactive maintenance and timely repairs reduce downtime and safety risks, ensuring reliable, efficient furnace operation throughout the heating season.