Why a Furnace Keeps Trying to Ignite and How to Fix It

The problem of a furnace that keeps trying to ignite can be frustrating, noisy, and potentially unsafe. This article explains common causes, step-by-step troubleshooting, repair options, cost expectations, and preventive maintenance to help homeowners diagnose and address the issue safely and effectively. Key takeaway: many ignition cycling issues are caused by dirty sensors, faulty ignition components, airflow problems, or control faults and can often be resolved with targeted repairs or professional service.

Symptom	Likely Cause	Possible Action
Repeated ignition attempts with no flame	Faulty ignitor, gas valve, or gas supply	Visual inspection; replace ignitor or call technician
Ignitor glows but flame fails	Blocked burners, faulty gas valve, or gas pressure	Clean burners; check gas pressure; professional service
Ignitor clicks repeatedly	Relays, control board, or thermostat issue	Test control board and wiring; replace faulty parts
Short ignition cycles with frequent starts/stops	Flame sensor contamination or limit switch trip	Clean flame sensor; check airflow and filters

How Gas Furnace Ignition Systems Work

Understanding the basic ignition sequence helps identify where problems occur. When the thermostat calls for heat, the furnace control board powers the inducer motor, opens the gas valve, and commands the ignitor to create a spark or glow. The burners light and the flame sensor confirms combustion, allowing the gas valve to remain open. If the flame sensor doesn’t confirm a flame or a safety device detects a fault, the control board shuts the gas off and retries the sequence.

Common Reasons A Furnace Keeps Trying To Ignite

Dirty Or Faulty Flame Sensor

The flame sensor tells the furnace that burners are lit. If it’s coated with soot or corrosion, the control board may not detect flame and will shut off the gas, causing repeated ignition attempts. Cleaning the flame sensor is a frequent, low-cost fix that often resolves cycling issues.

Weak Or Broken Ignitor

Modern furnaces use hot surface ignitors or spark ignitors. A cracked, weak, or intermittently failing ignitor may glow or spark without reliably igniting gas. This causes the control board to retry ignition. Ignitors are fragile and require careful replacement when defective.

Gas Supply Problems Or Faulty Gas Valve

If gas pressure is low or the gas valve fails to open properly, the ignitor may operate but burners won’t light. This leads to repeated attempts and safety lockouts. Gas supply issues can originate from utility interruptions, a closed valve at the meter, or a failing internal gas valve.

Clogged Burners Or Pilot Orifice (Older Furnaces)

Dirt and debris on burners can prevent proper gas flow and ignition. In older pilot-based systems, a dirty pilot orifice produces weak flame. Burners that are misaligned or clogged cause spotting and unreliable ignition attempts.

Control Board, Relay, Or Transformer Faults

The furnace control board coordinates ignition timing and safety checks. Malfunctioning relays, worn wiring, or a failing transformer can interrupt signals, causing repeated ignition attempts or erratic behavior. The control board may also falsely sense problems and retry cycles unnecessarily.

Thermostat Or Wiring Issues

A faulty thermostat or loose wiring can send intermittent heat calls, causing the furnace to start and stop repeatedly. Shorted thermostat wiring may create rapid on/off cycles that look like ignition retrying but are actually repeated calls from the thermostat.

Blocked Venting Or Inducer Motor Problems

Exhaust vent blockages, a failing inducer motor, or pressure switch issues may prevent the furnace from establishing proper draft. The control board often retries ignition when pre-start safety checks fail.

Safety First: When To Stop And Call A Professional

If natural gas odor, strong smell of combustion, or visible gas leaking is present, evacuate the area immediately and call the gas company and emergency services. Repeated ignition attempts can increase risk if gas accumulates. When in doubt about gas-side components, always call a licensed HVAC technician.

Step-By-Step Troubleshooting For Homeowners

Turn Off Power And Gas Before Any Inspection

Always switch off furnace power at the breaker and shut off the gas supply before opening panels. This prevents shock and gas hazards.

Inspect Error Codes And Furnace Behavior

Many furnaces flash LED codes on the control board; consult the owner’s manual or manufacturer chart. Recording the exact pattern of lights and timing of attempts can save diagnostic time for a technician.

Check Air Filter And Airflow

A severely dirty filter or blocked return vents can cause high heat and limit-switch trips. Replace or clean the filter, ensure vents are open, and verify airflow at supply registers.

Clean The Flame Sensor

Remove and gently clean the flame sensor with fine-grit sandpaper or steel wool to remove oxidation. Reinstall the sensor and test the furnace. Do not use harsh chemicals and handle the sensor by the base to avoid damage.

Inspect The Ignitor

With the power off, inspect the ignitor for cracks or visible damage. Hot surface ignitors are brittle; if there is any fracture the part should be replaced. Ignitor replacement is a common DIY for competent homeowners, but it requires careful handling.

Examine Burners And Flame Pattern

After restoring power and gas, observe the burners during operation. Flames should be steady and blue with minimal yellow tips. Yellow, lifting, or irregular flames indicate combustion or airflow issues and warrant professional service.

Check For Obvious Gas Supply Problems

Confirm the gas valve near the furnace and the main shutoff are open. If other gas appliances are also weak, contact the gas utility. Do not attempt to repair gas piping unless certified.

Look For Blocked Vents Or Exhaust

Check outdoor vents and the chimney for debris, nests, or ice. Obstructed venting can trip pressure switches and cause repeated ignition attempts while the furnace tries to establish safe draft.

When To Replace Parts Vs. Call A Technician

Homeowners can often clean flame sensors, replace filters, and swap ignitors if comfortable with basic mechanical tasks. Gas valve replacement, control board diagnosis, pressure testing, and gas piping work should be performed by a licensed HVAC or plumbing professional due to safety and code requirements.

Typical Repair Costs And Timeframes

Costs vary by region and furnace model. Typical national averages: flame sensor cleaning or replacement $75–$250; ignitor replacement $100–$400; gas valve replacement $300–$900; control board replacement $250–$800. Simple fixes take 30–90 minutes; complex diagnostics and parts replacement may require several hours or a return visit.

Preventive Maintenance To Reduce Ignition Cycling

• Annual professional tune-up: cleaning burners, checking gas pressure, testing safety switches, and calibrating controls.
• Replace or clean filters every 1–3 months depending on use and pets.
• Keep vents and outdoor pipes clear of debris, snow, and nests.
• Monitor for early signs: unusual noises, repeated short cycles, or unsteady flames.

Common Diagnostic Tests Technicians Use

Technicians use multimeters to test continuity of ignitors, check voltage to gas valves, monitor flame-sensing millivolt signals, and read control board diagnostics. Combustion analyzers measure CO and efficiency. Pressure gauges confirm proper gas supply. These tests pinpoint whether the problem is electrical, gas-related, or mechanical.

Replacing Major Components: Tips And Notes

Ignitor Replacement

Choose OEM or compatible replacement parts and handle hot surface ignitors carefully. Ensure the unit is powered down, photograph connections for reassembly, and test operation after replacement.

Flame Sensor Replacement

New flame sensors are inexpensive and simple to swap. If cleaning restores function temporarily but problems recur, replacement is recommended.

Control Board Replacement

Control board failures may present as intermittent or erratic ignition attempts. Boards should be matched precisely to the furnace model and installed by a technician because programming or calibration may be required.

Frequently Asked Questions

Why Does The Furnace Try To Ignite Several Times And Then Lock Out?

Most furnaces perform multiple ignition attempts if the flame is not detected. After a preset number of failures the control board enters a lockout to prevent unsafe gas accumulation. Resetting the furnace may clear symptoms temporarily but underlying issues need diagnosis.

Can A Dirty Air Filter Cause Ignition Problems?

A very dirty filter can cause overheating and limit switch trips, which interfere with normal ignition cycles. While not a direct cause of failed ignition, poor airflow exacerbates safety trips and cycling behavior.

Is It Safe To Reset The Furnace Repeatedly?

Occasional reset after a resolved issue is acceptable, but repeated resets hide ongoing problems and increase wear. If resets are needed frequently, schedule professional service.

How To Choose A Qualified HVAC Technician

Look for licensed, insured HVAC contractors with positive local reviews and experience on the specific furnace brand. Request written estimates, ask about parts warranties, and confirm that technicians perform a safety check and combustion test after repairs.

Key Takeaways

A furnace that keeps trying to ignite is commonly caused by flame sensor contamination, ignitor failure, gas supply or valve issues, control board faults, or airflow and venting problems. Many issues are diagnosable with basic inspections and cleaning, but gas-side problems and electrical control repairs should be handled by a licensed technician to ensure safety and code compliance.