Furnace Ignitor Glows Then Shuts Off — Causes and How to Fix

The furnace ignitor glowing briefly and then shutting off is a common heating problem that leaves homes cold and owners confused. This article explains the most likely causes, diagnostic steps, safety precautions, and practical fixes to restore reliable furnace operation. Readers will learn when to attempt DIY troubleshooting and when to call a qualified HVAC technician.

Symptom	Possible Causes	Recommended Action
Ignitor Glows Then Goes Out	Flame Failure, Gas Valve Issue, Flame Sensor, Control Board	Check error codes, clean/replace sensors, test gas flow, call tech
Ignitor Fails To Ignite	Weak Ignitor, Electrical Fault, No Gas	Test ignitor resistance, inspect wiring, verify gas supply
Intermittent Operation	Thermostat, Limit Switch, Draft Problems	Inspect controls, vents, and safety switches

How The Furnace Ignition Process Works

Modern gas furnaces typically use a hot surface ignitor or an intermittent pilot system to light the burners. The thermostat calls for heat, the furnace control board powers the ignitor, the ignitor heats and glows, the gas valve opens, and burners light. A flame sensor confirms combustion and the blower runs once heat is established.

When the ignitor glows then shuts off before the burner lights, the failure can occur at any point in this sequence: ignitor, gas supply/valve, flame confirmation, or control electronics.

Common Causes Of An Ignitor That Glows Then Shuts Off

1. Flame Sensor Contamination Or Failure

The flame sensor detects flame and tells the control board to keep gas flowing. A dirty or failing flame sensor will not register a flame quickly, causing the control board to shut the gas valve after the ignitor warms, resulting in the ignitor turning off.

Indicators: Furnace runs through ignition cycle, gas flows briefly, then system locks out or repeats cycle.

2. Gas Valve Or Gas Supply Problems

If the gas valve does not open or gas pressure is low, the ignitor may glow but burners will not light. The control board senses no flame and shuts down the sequence. Issues include blocked gas line, faulty valve, or pilot gas pressure problems in some systems.

Indicators: Smell of gas (do not try to light), no burner ignition even with a healthy ignitor, multiple failed attempts to light.

3. Faulty Ignitor Or Weak Ignitor Output

Ignitors degrade over time. A hot surface ignitor may glow but fail to reach the temperature needed to ignite gas fully, or it can crack and break intermittently. Electrical supply to the ignitor can also be insufficient due to wiring or control board issues.

Indicators: Visible cracks on ceramic ignitor, high resistance when tested with a multimeter, delayed or dim glow.

4. Control Board Or Sequencer Faults

The furnace control board times and coordinates the ignition sequence. If it sends incorrect signals or prematurely cuts power to the ignitor or gas valve, ignition will fail. Control board faults can be intermittent and harder to diagnose.

Indicators: Random ignition failures, error codes on the control board, multiple unrelated components failing.

5. Thermostat Or Safety Switch Interruptions

Faulty thermostat wiring, a tripped limit switch, pressure switch failure, or vent blockage can interrupt the ignition sequence. Safety switches are designed to protect against unsafe conditions and will shut the system down if triggered.

Indicators: Furnace runs briefly then shuts off, error codes for limit/pressure switches, issues tied to airflow or draft.

Diagnostic Steps To Find The Root Cause

Follow safety-first diagnostics: turn off power and gas before handling components, never bypass safety devices, and call a pro for gas-related work. The following steps help pinpoint the issue while minimizing risk.

Visual Inspection And Error Codes

Check the furnace access panel for an LED status code or error display. Consult the furnace manual to decode blinking patterns. Inspect the ignitor for cracks, soot, or discoloration and examine wiring for loose or burnt connections.

Test The Ignitor

Use a multimeter to measure the ignitor’s resistance (for hot surface ignitors) and compare with manufacturer specs. A very high or infinite resistance typically indicates a broken ignitor that needs replacement. Observe the ignitor glow: dim, delayed, or uneven glow suggests replacement.

Check The Flame Sensor

Remove the flame sensor and gently clean it with fine-grit sandpaper or steel wool to remove oxidation or soot. Reinstall and test. If cleaning doesn’t help, test the flame sensor with a multimeter or replace it; these sensors are relatively inexpensive.

Verify Gas Supply And Valve Operation

Confirm the home’s gas supply is on and other gas appliances work. A licensed technician should test gas pressure and valve operation. Never attempt gas valve repairs without certification. For pilot-equipped systems, ensure pilot flame is strong and properly positioned against the thermocouple.

Inspect Pressure And Limit Switches

Check the venting and draft inducer for blockages or failures. A stuck pressure switch or tripped limit switch can stop the ignition sequence. Clean vents and ensure the inducer motor runs during startup. Replace faulty switches per the manufacturer’s guidance.

Examine Control Board And Wiring

Inspect control board connections for burnt spots, corrosion, or loose terminals. Look for error codes that point to control board faults. If the board appears damaged or error diagnostics are inconsistent, replacement or professional repair may be required.

DIY Fixes Homeowners Can Safely Attempt

Clean The Flame Sensor

Turn off power to the furnace, remove the sensor, gently clean with fine-grade sandpaper, wipe with a clean cloth, and reinstall. This fix resolves many “glow then shut off” cases caused by sensor contamination.

Reset The Furnace And Check Filters

Reset the system by turning off power for one minute and restoring it. Replace dirty air filters and clear any vent obstructions. Restricted airflow can trigger safety shutoffs that interrupt ignition cycles.

Check Thermostat Settings And Wiring

Confirm the thermostat is set to heat and communicating correctly. Replace batteries in a battery-powered thermostat and ensure wiring terminals are tight. Faulty thermostat signals can cause premature shutdown during ignition.

When To Call An HVAC Professional

Call a licensed HVAC technician for any task involving gas valves, pressure testing, control board replacement, or if diagnostic steps do not resolve the issue. Also seek professional help if there is any smell of gas, persistent error codes, or safety switch trips.

Professional services include: gas valve replacement, control board diagnostics and replacement, pressure and combustion testing, safe ignitor replacement, and thorough venting and heat exchanger inspection.

Parts Replacement Guide And Typical Costs

Component	Typical Cost (Parts Only)	Notes
Flame Sensor	$20–$80	Easy DIY replacement for many models
Ignitor (Hot Surface)	$30–$150	Fragile; handle carefully, professional fit recommended
Gas Valve	$150–$500	Professional replacement required
Control Board	$100–$600	Match part number; may require programming
Pressure/Limit Switch	$20–$120	Often straightforward replacement

Safety Precautions And Best Practices

Never bypass flame sensors, limit switches, or other safety devices. Bypassing creates a risk of gas leaks or unburned gas buildup and can cause carbon monoxide hazards. If the home smells of gas, immediately shut off the gas at the meter, leave the house, and call the utility or 911.

Schedule annual professional maintenance to reduce ignition problems. Routine checks include cleaning flame sensors, inspecting ignitors, testing gas pressure, checking heat exchangers for cracks, and verifying safe venting.

Preventive Maintenance Tips To Reduce Recurrence

• Change air filters every 1–3 months to maintain airflow and prevent limit switch trips.
• Keep the furnace area clean and free of dust and debris to reduce sensor contamination.
• Have an HVAC technician perform annual tune-ups before heating season.
• Ensure proper venting and clearance around intake and exhaust to prevent pressure switch issues.
• Replace aging components proactively—ignitors and sensors have finite lifespans.

Frequently Asked Questions

Is It Safe To Run The Furnace If The Ignitor Keeps Shutting Off?

It is not recommended to repeatedly run the furnace with ignition failures. Failed ignition attempts increase wear and may indicate gas or safety issues. Repeated failures should prompt professional inspection.

How Long Does A Typical Ignitor Last?

Hot surface ignitors generally last 3–10 years depending on usage, model quality, and electrical cycling. Frequent cycling or voltage spikes shorten lifespan.

Can A Dirty Filter Cause The Ignitor To Turn Off?

Yes. A severely clogged filter restricts airflow, causing heat buildup and tripping limit switches that interrupt the ignition sequence. Regular filter changes help avoid this scenario.

Key Troubleshooting Checklist

1. Check furnace error codes and consult the manual.
2. Visually inspect the ignitor for cracks and test resistance with a multimeter.
3. Clean or replace the flame sensor.
4. Verify gas supply and look for gas smells; do not attempt valve repairs if gas is suspected.
5. Inspect pressure, limit switches, and venting for blockages or failures.
6. If the problem persists, call a licensed HVAC technician for safe diagnosis and repair.

This article equips homeowners with the knowledge to understand why a furnace ignitor glows then shuts off, how to perform safe basic diagnostics, and when to engage a professional. Proper diagnosis and timely action restore reliable heating while keeping safety the top priority.