The article explains why a furnace burner may not ignite, how to troubleshoot safely, and when to call a professional. It covers common causes like faulty ignitors, gas supply issues, flame sensors, pilot problems, and control board failures, providing step-by-step diagnostics and realistic cost estimates for repairs.
| Problem | Likely Cause | Quick Fix |
|---|---|---|
| Clicking But No Flame | Ignitor Or Gas Valve | Inspect/Replace Ignitor; Check Gas Supply |
| No Clicking, No Flame | No Power Or Broken Transformer | Check Breaker, Thermostat, Transformer |
| Burners Ignite Then Go Out | Dirty Flame Sensor Or Vent Blockage | Clean Sensor, Check Venting |
| Pilot Won’t Stay Lit | Pilot Assembly, Thermocouple | Clean Pilot, Replace Thermocouple |
Why Burners Fail To Ignite
When a furnace burner does not ignite, the issue can stem from electrical, gas, mechanical, or safety systems. Common causes include a failed ignitor, blocked gas line, faulty flame sensor, bad thermostat, or a tripped safety switch. Understanding these categories helps pinpoint troubleshooting steps.
Safety First: Before Any Troubleshooting
Always prioritize safety. Turn off power to the furnace at the breaker and shut off the gas if performing internal checks. Work in a well-ventilated area and avoid attempting repairs involving gas lines unless qualified. If gas smell is present, evacuate and call the gas company or emergency services immediately.
Basic Checks Before Opening The Furnace
Start with simple checks that often solve ignition problems without needing to open the unit.
- Thermostat Settings: Confirm the thermostat is set to heat, with a temperature above current room temp and correct mode (heat, not fan only).
- Power Supply: Verify the furnace breaker is on and any switch near the furnace is in the ON position.
- Pilot Light: For older furnaces with a pilot, check if the pilot is lit; relight only according to the manufacturer’s instructions.
- Gas Supply: Ensure the home’s gas supply is active and other gas appliances work. If no gas, contact the gas provider.
- Air Filter: Replace clogged filters; extreme restriction can trigger safety limits and prevent ignition.
Ignitor Problems: Electronic Ignition And Hot Surface Ignitors
Modern furnaces use hot surface ignitors (HSI) or intermittent pilots (spark ignitors). A cracked or worn ignitor is a top cause of non-ignition and should be inspected visually.
Symptoms Of Ignitor Failure
Typical signs include a repeated clicking without flame or the furnace attempting to light but failing. An HSI often glows orange then breaks; if it doesn’t glow, it’s likely faulty.
Testing And Replacement
Testing requires the power to the furnace and a multimeter to check continuity for HSI types. If the ignitor shows open circuit or visible cracks, replacement is the recommended fix. Replacing an ignitor is a common DIY for handy homeowners but must be done carefully due to the fragility of the component and the need to disconnect power and gas interlocks.
Flame Sensor Issues
The flame sensor detects combustion. If it’s dirty, the control board may shut the gas off shortly after ignition, producing a cycle where burners light then go out.
Cleaning The Flame Sensor
Remove the sensor and gently clean it with fine-grit sandpaper or steel wool to remove oxidation. Reinstall and test. Cleaning often restores normal operation and is an inexpensive first step.
When To Replace
If cleaning fails, the sensor or the wiring to it may be defective and should be replaced. Sensors are inexpensive, and replacement is straightforward for a trained DIYer.
Pilot Assembly And Thermocouple Problems (Older Furnaces)
Older systems with standing pilots rely on the thermocouple to sense a lit pilot and keep the gas valve open. A weak or dirty thermocouple can cut gas flow, preventing burners from igniting.
Relighting And Cleaning
Relighting the pilot per manufacturer instructions and cleaning the pilot orifice can resolve issues. If the pilot fails to stay lit, test the thermocouple for millivolt output. Replace if it fails to produce sufficient voltage.
Gas Valve And Supply Issues
The gas valve controls fuel flow to burners. If the valve is defective or the supply pressure is insufficient, the burners won’t light even if the ignitor works.
How To Narrow Down Gas Problems
Confirm other gas appliances operate. If they do, the problem is likely the furnace gas valve, regulator, or internal piping. Professional technicians use manometers to verify gas pressure and inspect the valve solenoid and controls.
Repair Considerations
Gas valve replacement requires a licensed technician due to code and safety requirements. Expect higher labor costs because of gas work and regulatory inspections in some jurisdictions.
Control Board, Transformers, And Electrical Failures
Modern furnaces depend on control boards and low-voltage transformers. A failed control board, relay, or transformer can prevent ignition sequence signals from reaching the ignitor or gas valve.
Diagnostic Steps
Check for error codes on the furnace LED—many units flash diagnostic codes that indicate the problem. Measure transformer output to ensure proper low-voltage supply. Resetting the power can clear some faults but persistent faults indicate component replacement.
Blocked Venting And Combustion Air Restrictions
High-efficiency condensing furnaces require clear intake and exhaust pipes. Blocked vents or frozen condensate traps can trigger safety switches preventing ignition.
Common Causes And Fixes
Leaves, nests, or snow can block PVC venting. Clear obstructions and inspect the condensate line. Ensure intake air from the basement or utility closet is unobstructed. If the inducer motor is failing, burners may not ignite.
Limit Switches And Safety Interlocks
Limit switches prevent overheating and will shut off the burners if tripped. A stuck or faulty limit switch stops ignition to protect the furnace.
Testing And Reset
Check for tripped high-limit conditions—poor airflow or blocked return ducts often cause repeated tripping. Replace failed switches and fix airflow issues to prevent recurrence.
Step-By-Step Troubleshooting Checklist
- Confirm thermostat settings and power to furnace. Replace batteries in thermostat if present.
- Inspect the furnace filter and replace if dirty.
- Look for visible ignitor damage or flame sensor buildup.
- Check for error codes on the furnace control board LED and consult the manual.
- Listen for inducer motor and ignitor sounds; note any clicks or attempts to light.
- Smell for gas; if present, leave and call emergency services.
- Verify gas supply to the home; check other gas appliances.
- Clean the flame sensor and pilot assembly if applicable.
- Call a licensed HVAC technician if the issue involves gas valve, control board, or sealed combustion components.
When To Call A Professional
Call a licensed HVAC technician if troubleshooting points to gas valve replacement, control board failure, persistent electrical faults, or blocked venting that requires roof or exterior access. Professionals are also necessary when local codes mandate licensed work for gas line repairs or appliance replacements.
Cost Estimates For Common Repairs
These are ballpark U.S. ranges; actual prices vary by region and furnace model.
| Repair | Parts Cost | Labor Cost |
|---|---|---|
| Ignitor Replacement | $30–$150 | $100–$300 |
| Flame Sensor Cleaning/Replace | $10–$80 | $80–$200 |
| Thermocouple Replacement | $10–$50 | $75–$200 |
| Gas Valve Replacement | $150–$500 | $200–$600 |
| Control Board Replacement | $100–$600 | $150–$500 |
Preventive Maintenance To Reduce Ignition Failures
Regular maintenance reduces ignition issues. Recommended tasks include annual professional tune-ups, filter replacement every 1–3 months, cleaning flame sensors, keeping vents clear, and scheduling combustion analysis for gas appliances.
- Annual Tune-Up: Technician inspects ignition components, cleans burners, checks gas pressure, and verifies safety sensors.
- Filter Schedule: Replace disposable filters monthly during heavy use seasons; higher-efficiency filters may need more frequent checks due to airflow restrictions.
- Keep Vents Clear: Maintain 2–3 feet clearance around intake and exhaust terminations to prevent obstruction from leaves, snow, or debris.
Troubleshooting FAQs
Why Does The Furnace Click But Not Light?
Clicking without ignition often indicates the ignitor is attempting to start but either the gas valve isn’t opening or the ignitor is failing. Check for ignitor glow and verify gas flow.
Why Do Burners Light Then Go Out?
If burners light briefly but then shut off, a dirty flame sensor or safety interlock is likely. Cleaning the sensor typically resolves this symptom.
Is It Safe To Replace An Ignitor Myself?
Replacing an ignitor can be safe for competent DIYers if power is disconnected and care is taken with fragile components. For gas valve or control board work, hiring a licensed technician is advised.
Key Takeaways
Burner non-ignition typically falls into a few common categories: ignition component failure, gas supply issues, flame detection problems, electrical/control faults, and venting or safety interlocks. Systematic troubleshooting—starting with power, thermostat, and filters—often identifies the issue. For any work involving gas valves, regulators, or complex electrical components, a licensed HVAC technician should be engaged.