Why a Furnace Burns but Won’t Ignite Burners and How to Fix It

The furnace not igniting burners can leave a home without heat and create safety concerns. This article explains common causes, diagnostic steps, and repair options for a furnace whose burners fail to ignite, with clear troubleshooting guidance and when to call a technician. Key topics: ignition systems, pilot vs. electronic ignition, common faults, safety checks, and preventative maintenance.

Issue	Likely Cause	Initial Action
Clicking/No Flame	Faulty igniter or transformer	Inspect igniter visually, check error codes
Pilot Won’t Light	Thermocouple or gas valve problem	Check thermocouple position, test gas supply
Intermittent Ignition	Dirty burners or flame sensor	Clean burners and flame sensor
Gas Smell	Gas leak or stuck valve	Evacuate and call gas company

How Furnace Ignition Systems Work

Understanding ignition basics, the furnace igniter initiates combustion by creating a spark or heating element to light the gas delivered to the burners. Two common systems are standing pilot with a thermocouple and electronic ignition using spark or hot-surface igniters. Modern furnaces typically use electronic ignition for higher efficiency and safety.

Standing Pilot: A small pilot flame remains lit and the thermocouple senses heat to keep the gas valve open. Electronic Ignition: Includes intermittent pilot, spark ignition, or hot-surface igniters that light only when heat is needed, reducing gas use.

Common Causes Why Burners Won’t Ignite

Many problems can prevent burners from igniting. Identifying the symptom narrows the diagnosis: no clicking sound, clicking without a flame, pilot lighting issues, or gas odor. The most frequent causes include ignition component failure, safety interlocks, gas supply problems, dirty burners, and electrical faults.

Igniter Failure

Hot-surface igniters (ceramic bars) and spark igniters degrade over time. A cracked or weak hot-surface igniter will glow dimly or not at all, and spark electrodes may corrode or become misaligned. Replacement is often necessary when electrical tests show open circuits or visible damage.

Faulty Flame Sensor

The flame sensor confirms combustion; if fouled or failing, it shuts the gas valve after ignition to prevent unmonitored gas flow. A corroded sensor typically results in the burners lighting briefly then shutting off repeatedly. Cleaning often restores function unless the sensor is damaged.

Gas Valve Or Supply Issues

A closed or defective gas valve, insufficient supply pressure, or an interrupted gas line prevents burners from lighting. If the furnace receives a call for heat but no gas reaches the burners, the system cannot ignite. A professional should test gas pressure and valve operation for safety.

Pilot And Thermocouple Problems

In furnaces with a standing pilot, a weak pilot flame or misaligned thermocouple will fail to keep the gas valve open. The thermocouple must remain heated by the pilot flame; otherwise the safety mechanism shuts off gas flow to the burners.

Safety Switches And Limit Switches

Blocked flues, dirty filters, or failed pressure switches and limit switches can prevent ignition. The system uses these interlocks to avoid unsafe conditions; if they detect an issue, the control board will inhibit the ignition sequence until resolved.

Diagnostics: What To Check First

Before calling a technician, homeowners can perform safe basic checks. Always follow manufacturer instructions and turn off power when inspecting internal components. If a gas odor is present, evacuate and call the gas utility immediately.

• Thermostat Settings: Confirm the thermostat is set to heat and the setpoint is above current room temperature.
• Power Supply: Verify the furnace has power—check the breaker and furnace switch.
• Gas Supply: Ensure the gas valve at the furnace is open and other gas appliances are working if safe to check.
• Air Filter And Venting: Replace clogged filters and confirm vents and flue are clear.
• Error Codes: Inspect the furnace control board LED for flash codes; consult the manual to interpret them.

Step-By-Step Troubleshooting

1. Observe The Ignition Sequence

Turn up the thermostat and watch the furnace start. Note sounds—clicking indicates attempted ignition, a roar may indicate delayed ignition, while silence may point to control or power issues. Recording the sequence helps when speaking with a technician.

2. Inspect The Igniter And Electrodes

For hot-surface igniters, visually check for cracks or breaks. For spark systems, inspect electrodes for soot or corrosion. Use a multimeter to test continuity; a failed igniter typically shows infinite resistance.

3. Clean The Flame Sensor

Shut power to the furnace, remove the flame sensor, and gently clean the metal rod with fine steel wool or emery cloth. Reinstall and test the system. Cleaning often resolves short cycling or shutdown after ignition.

4. Check The Pilot And Thermocouple

If the furnace has a pilot light, confirm the pilot is steady and in proper contact with the thermocouple. Replace the thermocouple if it is bent, damaged, or fails to produce millivoltage when heated.

5. Evaluate Gas Valve And Pressure

Only trained technicians should test gas pressure and internal valve operation. If basic checks suggest a gas flow issue—no hiss or flame and other appliances are affected—contact a professional or the gas utility.

When Flames Ignite But Then Go Out

If burners light briefly and then shut off, look to the flame sensor, draft inducer, or limit switch. The sensor may not detect the flame due to dirt or mispositioning. A failing inducer motor or clogged vent can set a pressure switch that stops the gas flow after ignition.

Safety Precautions And When To Stop

If a gas smell, unusual noises, or persistent ignition attempts occur, turn off power and gas to the furnace and ventilate the area. Evacuate if a strong odor exists and contact the gas company immediately. Electrical work and gas valve repairs should be left to licensed HVAC or gas technicians.

Repair Options And Estimated Costs

Repair costs vary by component and region. Typical price ranges include igniter replacement ($100–$300 parts and labor), flame sensor cleaning or replacement ($75–$180), thermocouple replacement ($50–$150), and gas valve replacement ($300–$700). Complex control board or inducer motor repairs may cost more.

Component	Typical Cost Range	Notes
Hot-Surface Igniter	$100–$300	Fragile; handle carefully during replacement
Flame Sensor	$75–$180	Often fixed by cleaning; replacement if damaged
Thermocouple	$50–$150	Common for older systems with standing pilot
Gas Valve	$300–$700	Safety-critical; require licensed technician

Maintenance To Prevent Ignition Problems

Regular preventive maintenance reduces ignition failures. Annual professional tune-ups, filter changes every 1–3 months, keeping vents and flues clear, and occasional inspection of ignition components extend furnace life and reliability.

• Annual Inspection: Have a licensed HVAC technician perform a combustion analysis, safety checks, and component inspections.
• Filter Routine: Replace or clean filters to maintain airflow and prevent overheating that can trigger limit switches.
• Component Cleaning: Clean burners, flame sensors, and accessible ignition parts during service to prevent fouling.

When To Call A Professional

Contact a qualified HVAC technician or licensed gas professional for gas leaks, persistent ignition failures, control board errors, or whenever repairs involve gas valves, wiring, or pressure testing. Professionals carry the tools and certifications to ensure safe and compliant repairs.

How To Choose A Qualified Technician

Look for licensed HVAC contractors with positive local reviews, NATE certification, and proper insurance. Ask for written estimates, warranties on parts and labor, and evidence of training on the furnace brand. For gas issues, confirm the technician is certified to work with gas lines.

Useful Troubleshooting Checklist

1. Confirm thermostat call for heat and power to furnace.
2. Check circuit breaker and furnace switch.
3. Inspect air filter and replace if dirty.
4. Listen for ignition attempts and note any error codes.
5. Gently clean the flame sensor if burners light then shut off.
6. Visually inspect igniter for damage; do not touch hot-surface elements.
7. Ensure gas supply valve is open and other gas appliances function.
8. If a gas odor is present, evacuate and call gas utility immediately.

Key Takeaways For Homeowners

Prompt action matters: Delaying repairs can lead to costlier damage and risk. Simple fixes like cleaning a flame sensor or replacing a filter may restore function, but gas and electrical components require professional attention. Regular maintenance is the best defense against a furnace not igniting burners.

When symptoms are unclear or a gas smell exists, prioritize safety and contact a licensed technician. Accurate diagnosis, timely repair, and routine maintenance keep heating systems safe and reliable.