Furnace Ignites Then Turns Off: Causes, Diagnostics, and Fixes

The problem of a furnace igniting then turning off can be frustrating, uncomfortable, and potentially unsafe. This article explains the most common causes, diagnostic steps, and practical solutions for a furnace that lights briefly then shuts down, helping homeowners and technicians identify and address the issue efficiently.

Symptom	Common Causes	Likelihood
Ignites Then Shuts Off Quickly	Dirty flame sensor, clogged filter, blocked vent, faulty limit switch	High
Ignites, Runs Briefly, Then Resets	Pressure switch failure, inducer motor issue, gas valve problems	Medium
Ignites, Flame Flickers, Shuts Down	Thermocouple/thermopile (on older systems), gas supply fluctuations	Medium

How Furnaces Normally Operate

A typical gas furnace follows a sequence: the thermostat calls for heat, the inducer motor starts, the system performs safety checks via pressure and limit switches, the ignitor lights the burners, the flame is sensed, and the blower circulates warm air. Each component must perform correctly for the furnace to continue running.

Main Causes Why A Furnace Ignites Then Turns Off

Dirty Or Faulty Flame Sensor

The flame sensor verifies burner ignition. When it’s dirty or failing, it may not detect flame consistently and will shut the gas valve off quickly as a safety measure. This is one of the most common reasons for a furnace that ignites then turns off.

Restricted Airflow: Dirty Filter Or Blocked Vents

Restricted airflow can cause overheating and trigger the high-limit switch to shut down burners to prevent damage. A clogged filter, closed supply vents, or obstructed return registers are frequent culprits.

Limit Switch Or Safety Switch Problems

The limit switch monitors heat exchanger temperatures. If it trips due to overheating or a fault, it will cut gas flow. Repeated trips often indicate airflow problems, a failing switch, or a dirty heat exchanger.

Pressure Switch And Inducer Motor Issues

The pressure switch confirms proper venting via the inducer motor. If the inducer doesn’t reach required vacuum levels because of a weak motor, blocked vent, or faulty switch, the furnace may ignite briefly or not complete the pre-ignition checks, causing shutdowns.

Gas Supply And Gas Valve Problems

Fluctuating gas pressure, a partially closed service valve, debris in the gas line, or a failing gas valve can lead to brief ignition followed by shutdown. Insufficient gas can cause a weak flame that the flame sensor fails to read.

Ignitor Or Pilot/ Thermocouple Issues (Older Systems)

In direct spark or hot surface ignition systems, a failing ignitor may light intermittently. In older pilot-based systems, a weak pilot flame or failing thermocouple/thermopile can result in the pilot extinguishing or losing signal, causing the valve to close.

Control Board And Wiring Faults

Malfunctioning control boards, loose connectors, or intermittent wiring can interrupt the ignition sequence or flame sensing feedback, causing the furnace to shut off shortly after lighting.

Symptoms And What They Suggest

Furnace Lights, Runs For 10–30 Seconds, Then Shuts Off

This pattern often points to a dirty flame sensor, faulty flame rollout switch, or limit switch tripping from restricted airflow or a dirty heat exchanger.

Ignitor Clicks Or Glows But Burners Don’t Stay Lit

If the ignitor works but the burners extinguish, check the flame sensor, gas valve, and gas pressure. Also inspect the inducer and pressure switch sequence to ensure safe operation.

Blower Runs But Fire Shuts Off Immediately

When the inducer runs and the furnace starts the sequence but the flame doesn’t remain, suspect flame sensing issues or control board timing errors that terminate the gas flow.

Step-By-Step Diagnostic Checklist

1. Visual Inspection: Check vents, flue, and intake for obstructions like nests, debris, or snow. Inspect burners for soot or discoloration.
2. Check Air Filter And Vents: Replace dirty filters and ensure supply and return vents are open and unobstructed.
3. Read The Error Codes: Many furnaces have an LED diagnostic light on the control board. Record blink patterns and match to the manufacturer code chart.
4. Test The Flame Sensor: With power off, remove and gently clean the flame sensor rod with fine steel wool or emery cloth. Reinstall and test operation.
5. Inspect The Ignitor: For hot surface ignitors, look for cracks and test resistance per the manufacturer. Replace if cracked or out of spec.
6. Check Pressure Switch And Inducer: Listen for the inducer running and observe the pressure tubing for cracks. Verify the pressure switch closes during the inducer run.
7. Measure Gas Pressure: A pro should measure manifold and inlet pressures to verify fuel delivery is within spec.
8. Test Limit Switches: Use a multimeter to confirm continuity; replace if intermittent or out of specification.
9. Inspect Control Board And Wiring: Look for burnt traces, loose plugs, or corroded connectors and repair as needed.

Common DIY Fixes Homeowners Can Try

Replace The Air Filter

This simple step often fixes overheating-related shutdowns. Use the recommended MERV-rated filter and replace per manufacturer guidance.

Clean The Flame Sensor

Carefully remove the flame sensor and clean the metal rod. Do not use harsh abrasives or bend the sensor. Reinstall and observe if the furnace continues running normally.

Clear Vents And Intake

Remove visible debris from intake and exhaust vents outside. Ensure proper clearance from landscaping, snow, and stored items that may block airflow.

Reset The Furnace

Turn off power at the thermostat and furnace breaker for a few minutes, then restore power. This can clear transient faults, but recurring issues indicate deeper problems.

When To Call A Professional HVAC Technician

Call a licensed HVAC professional if the furnace continues to ignite then turn off after basic cleaning and filter changes, if gas smells are present, or if error codes indicate pressure switch, gas valve, or control board failures. Working on gas components, venting, and combustion systems requires training and certification for safety and code compliance.

Safety Precautions

• If a strong smell of gas is present, evacuate the home and call the gas company or 911 immediately.
• Turn off power to the furnace before performing inspections that require access to internal components.
• Do not attempt to repair gas valves, combustion controls, or sealed components without proper certification.

Maintenance Practices To Prevent Reoccurrence

Regular preventive maintenance reduces the chance of a furnace igniting then turning off unexpectedly. Annual professional tune-ups, seasonal filter changes, keeping vents clear, and prompt repairs of any detected issues help maintain reliable operation and efficiency.

Cost Estimates For Common Repairs

Repair	Typical U.S. Cost Range
Air Filter Replacement	$10–$60
Flame Sensor Cleaning/Replacement	$60–$200
Ignitor Replacement	$100–$300
Pressure Switch Or Inducer Motor Repair	$150–$600
Gas Valve Or Control Board Replacement	$300–$1,000+

Frequently Asked Questions

Is It Dangerous If The Furnace Keeps Shutting Off?

Short cycling itself stresses the system and reduces efficiency, but the built-in safety shutdowns prevent combustion hazards. Persistent cycle faults should be evaluated promptly to prevent component damage and potential safety risks.

How Long Can A Furnace Run After Cleaning A Flame Sensor?

Cleaning a flame sensor often resolves the issue immediately and can last many months, but recurring need for cleaning may indicate excess combustion residue or other combustion inefficiencies requiring professional diagnosis.

Can A Dirty Heat Exchanger Cause Short Cycling?

Yes. A dirty or obstructed heat exchanger can contribute to overheating and limit switch trips, causing the furnace to shut off soon after ignition.

Key Takeaways

Flame sensor contamination, restricted airflow, pressure switch/inducer failures, and gas delivery problems are the most common reasons a furnace ignites then turns off. Simple maintenance like replacing filters and cleaning the flame sensor often solves the problem, but gas system issues and control faults require a licensed HVAC technician. Regular maintenance and prompt attention to diagnostic codes keep heating systems safe and reliable.

For persistent issues, contact a qualified, licensed HVAC professional who can perform safe combustion testing, gas pressure checks, and certified repairs. Proper diagnosis prevents wasted energy, repeated outages, and potential safety hazards.