When a furnace burner lights but won’t stay lit, comfort and energy efficiency suffer, and safety concerns may arise. This guide explains common reasons a burner won’t remain lit, practical steps you can take, and when to call a licensed HVAC professional. By understanding flame sensors, ignition systems, gas supply, venting, and safety controls, homeowners can diagnose issues confidently and reduce wasted energy.
Causes Of A Burner That Won’t Stay Lit
A burner that won’t stay lit can stem from several intertwined issues. The most common culprits are a dirty or misaligned flame sensor, a faulty or worn ignition system, inconsistent gas pressure or a restricted gas valve, blocked venting or insufficient combustion air, and control problems such as a faulty limit switch or a malfunctioning control board. Identifying the right cause requires a systematic check of ignition and gas delivery, airflow, and safety switches.
- Dirty or misaligned flame sensor. A dirty sensor can fail to detect a flame, causing the system to shut down shortly after ignition.
- Ignition system faults. A weak or failing ignition source (electronic spark or hot surface igniter) may ignite briefly but fail to sustain flame.
- Gas pressure or valve issues. Low or fluctuating gas pressure or a partially closed valve can extinguish the flame.
- Venting or combustion air restrictions. Poor venting or insufficient fresh air can trigger safety shutoffs.
- Control or safety switch problems. A faulty limit switch, failed control board, or wiring fault can shut the burner off even after ignition.
Safety First: Immediate Steps If You Smell Gas Or The Burner Won’t Stay Lit
If you smell gas or suspect a gas leak, evacuate the area immediately, avoid any flames or sparks, and call the gas company or emergency services from a safe location. Do not restart the furnace or operate electrical switches in the area. If there is no odor and you can safely access the furnace, shut off the gas supply at the furnace shutoff valve and cut power to the furnace. Do not bypass safety devices; gas appliances require professional inspection after any suspected leak or ignition failure.
Once safety is ensured, document any symptoms (e.g., burner lights briefly, then goes out, or no ignition at all) and gather model information for the technician. If the flame goes out during normal operation, proceed with careful, systematic troubleshooting as outlined below.
Troubleshooting The Flame Sensor And Ignition System
The flame sensor and ignition system are common failure points for burners that won’t stay lit. Begin with the simplest checks and proceed to more involved steps if needed. Always power down the furnace before handling electrical components to avoid shock.
Inspect and clean the flame sensor. Locate the flame sensor near the burner. If it appears dirty or film-coated, gently remove oxidation with a soft abrasive pad or fine steel wool, then wipe with a cloth dampened with isopropyl alcohol. Reinstall and retest. A sensor that is misaligned or corroded may fail to sense flame and shut off the gas valve.
Check sensor alignment and connections. Ensure the sensor tip is in the correct position and that its wiring is intact and securely connected to the control board. Loose connections can cause intermittent flame sensing and shutoffs.
Evaluate the ignition source. For electronic ignition (spark or hot surface), listen for a healthy ignition spark or burner glow. If ignition is weak, misaligned, or delayed, the system may light but fail to sustain flame. Inspect ignition electrodes for cracks, buildup, or misalignment, and replace if necessary. Do not touch hot components.
Test after cleaning and inspection. After cleaning and verifying connections, power the furnace back on and attempt a heat call. If the burner still won’t stay lit, the issue may lie with the control board or gas valve, requiring professional testing with appropriate tools.
Check Gas Supply, Pressure, And Valves
A steady and correct gas supply is essential for reliable ignition. Start with the most basic checks and avoid tampering with gas lines if you are unsure.
Verify the gas shutoff valve is fully open. A partially closed valve restricts gas flow, causing the flame to extinguish. Ensure the valve lever is parallel to the pipe.
Inspect the gas line for kinks or leaks. A damaged or obstructed gas line can reduce pressure. Do not attempt to repair gas lines yourself; call a licensed technician if you suspect a leak or damage.
Assess the gas pressure. Improper pressure affects burner performance. If you suspect low pressure, contact the gas utility or a licensed HVAC technician to measure and adjust manifold pressure to the furnace’s specifications.
Watch for fluctuating supply. In some environments, different gas supplies or appliances may temporarily draw more gas, impacting the furnace. A technician can evaluate whether a dedicated line or pressure regulator is needed.
Inspect The Venting And Combustion Air
Ventilation and exhaust are critical for safe and stable combustion. Inadequate combustion air or blocked venting can cause flame instability or shutoffs.
Check for blockages in the vent pipe. Obstructions such as nests, leaves, snow buildup, or debris can hinder exhaust flow and trigger safety shutoffs. Clear any obstructions if you can do so safely, or call a professional to clear the vent system.
Inspect the air filter and return airflow. A dirty air filter reduces airflow, causing the furnace to overheat and trigger a limit switch that shuts off the burner. Replace or clean filters according to manufacturer guidelines, typically every 1–3 months during heating season.
Evaluate combustion air supply. Some homes require outdoor air or makeup air for proper combustion. If the house is unusually airtight, it may affect burner stability. A professional can determine if additional ventilation is needed.
Inspect Electrical Components: Thermostat, Control Board, And Limit Switch
Electrical issues can interrupt a successful ignition or cause the burner to shut down after ignition. Systematically check each component while the furnace is powered off for safety.
Verify thermostat calls for heat correctly. Ensure thermostat wires are connected, batteries are fresh (if applicable), and the thermostat setting matches the desired temperature. A faulty thermostat can fail to maintain a heat call, leading to short operation.
Inspect the control board and wiring harness. Look for visible signs of damage, loose connections, or burnt components. A faulty control board may intermittently cut ignition or fail to sustain flame.
Check the limit switch and safety controls. If the furnace overheats, the limit switch may shut off the burner to prevent damage. A reset or replacement may be necessary if it’s sticking or faulty. Do not bypass limit switches; they protect occupants and equipment.
When To Call A Pro: Professional Diagnostics And Repairs
Some furnace problems require specialized tools and knowledge. Call a licensed HVAC technician if any of the following apply: the burner still won’t stay lit after basic troubleshooting, you smell gas, there are unusual noises, or error codes appear on the furnace panel. A pro can safely test gas pressure, ignition voltages, flame rectification, and electrical signals, and perform precise adjustments or part replacements. Regular maintenance by a professional also helps prevent recurrence.
If you are uncertain about any step, avoid experimenting with gas lines, electrical connections, or venting. A professional assessment ensures safe operation and long-term reliability of the furnace.
Preventive Maintenance To Avoid Future Problems
Proactive maintenance helps reduce burner outages and extend furnace life. Implement these practices to keep the burner stable and efficient throughout the heating season.
- Schedule annual professional inspections. A technician will clean burners, inspect flame sensors, test ignition, and verify gas pressure and venting.
- Replace air filters regularly. Clean or replace filters every 1–3 months during heating season to maintain airflow and prevent overheating.
- Clean and inspect burners and igniters yearly. Remove soot, corrosion, and buildup; ensure proper alignment of igniters and flame sensors.
- Test safety controls periodically. Confirm that the thermostat, limit switch, and control board function correctly and safely respond to heat calls.
- Address unusual signs promptly. Unusual smells, noises, or inconsistent flame behavior warrant early professional evaluation to prevent more extensive damage.