Why a Furnace Won’t Ignite and Shuts Off: Troubleshooting Guide

The article explains common reasons a furnace won’t ignite and then shuts off, offering practical troubleshooting steps, safety checks, and guidance on when to call a professional. It focuses on gas and oil household furnace systems and provides diagnostic tips to help homeowners identify issues and make informed decisions about repairs or replacements.

Symptom	Likely Cause	Quick Fix
Clicking, No Flame	Igniter or pilot failure, gas valve issue	Inspect igniter; check gas supply; call pro
Flame Lights Then Dies	Dirty flame sensor, pressure switch, or vent blockage	Clean sensor; inspect venting; test safety switches
Short Run Cycle	Limit switch, thermostat error, or overheating	Check air filter; verify thermostat settings
Continuous Lockouts	Ignition control board or gas control malfunction	Reset system; replace control if persistent

How Furnace Ignition Systems Work

A basic understanding of how a furnace ignites helps diagnose why a furnace won’t ignite and then shuts off. Modern furnaces use either a standing pilot, hot surface igniter (HSI), or intermittent pilot/Direct Spark Ignition (DSI).

When the thermostat calls for heat, the ignition control initiates the ignition sequence, the gas valve opens, and the igniter lights the burners. A flame sensor or flame rectification system confirms flame presence; if the sensor doesn’t detect flame, the control will shut the gas valve to prevent unburned gas accumulation.

Common Causes When A Furnace Won’t Ignite Then Shuts Off

1. Faulty Flame Sensor

The flame sensor confirms the burners are lit. If it is dirty or failing, it may not detect flame and will cause the control board to shut the gas off after a few seconds. This is a frequent cause when the furnace lights then shuts off.

Typical sign: visible blue flame for a few seconds before the unit shuts down repeatedly.

2. Igniter Or Pilot Problems

Hot surface igniters can crack or fail with age; pilot lights can go out due to drafts or thermocouple failure. When the igniter fails to stay lit reliably, the furnace may cycle into lockout. Intermittent ignition failures often indicate an ignition component problem.

3. Gas Supply Or Gas Valve Issues

A restricted gas supply, a closed external gas shutoff, or a faulty gas valve can prevent proper ignition. The gas valve might open partially or fail after a brief moment, causing the flame to die out shortly after ignition.

4. Clogged Burners Or Dirty Components

Dirty burners, clogged orifices, or excessive soot in oil furnaces impede proper combustion. Restricted airflow or uneven gas distribution can cause weak flames that fail to sustain, triggering a shutdown.

5. Pressure Switch Or Venting Problems

High-efficiency furnaces rely on induction fans and pressure switches to confirm venting. A blocked flue, blocked condensate drain, or faulty pressure switch can cause the system to shut down during ignition attempts.

6. Limit Switch And Overheating

When the system overheats due to restricted airflow (dirty filter, closed vents) the limit switch opens to protect the furnace and stops the burners. Repeated short cycles where the unit ignites and then shuts off could indicate overheating and limit switch activation.

7. Faulty Ignition Control Board Or Wiring

The control board coordinates the ignition sequence. Faulty relays, damaged wiring, or sensor signals can interrupt the sequence mid-cycle and cause the gas valve to close as a safety response.

Initial Safety Steps Before Troubleshooting

Turn off power to the furnace and the gas supply before inspecting internal components. If a homeowner smells gas, evacuate the home, avoid using electronics, and call the gas company or 911 immediately.

Only perform visual and non-invasive checks unless comfortable with HVAC systems. For anything requiring gas, electrical, or combustion-system intervention, contact a licensed technician to avoid danger and code violations.

Step-By-Step Troubleshooting Guide

Step 1: Check Thermostat And Power

Verify thermostat settings are correct and the display is powered. Replace thermostat batteries and ensure the system is set to “Heat”. Check the furnace breaker and the switch near the furnace to confirm the system has power.

Step 2: Inspect Air Filter And Vents

A dirty filter restricts airflow and can cause short cycling or overheating. Replace clogged filters and ensure supply and return vents are open and unobstructed. This simple step resolves many short-run and shutdown scenarios.

Step 3: Observe The Ignition Sequence

Set the thermostat to call for heat and watch the sequence. Note whether the furnace attempts to ignite, whether the igniter glows or the pilot lights, and the exact timing of the shutdown. These observations narrow the likely causes.

Step 4: Clean The Flame Sensor

If the furnace lights then shuts off, remove and clean the flame sensor with fine emery cloth or steel wool. Reinstall carefully. A clean sensor frequently restores normal operation without parts replacement.

Step 5: Check The Igniter Or Pilot

For HSI systems, inspect the igniter for visible cracks. For pilot systems, check the pilot flame shape and the thermocouple. Replace the igniter or thermocouple if visibly damaged or if no flame is present.

Step 6: Inspect Venting And Pressure Switch

Clear any visible obstructions in intake or exhaust venting. Listen for the inducer fan and observe whether the pressure switch closes during the start-up sequence. A multimeter can confirm pressure switch continuity when the inducer runs.

Step 7: Evaluate The Gas Supply

Confirm nearby gas appliances operate and that the gas shutoff valve to the furnace is open. If gas supply is suspected limited, contact the gas utility or a licensed technician for pressure testing and valve inspection.

Step 8: Check The Limit Switch And Overheat Conditions

Inspect the limit switch and surrounding heat exchanger for signs of overheating, corrosion, or blockages. Replace the switch if it is tripping prematurely after ruling out airflow and venting problems.

Step 9: Look For Error Codes And LED Blink Patterns

Most modern furnaces display error codes via the control board LED. Count blink patterns and consult the furnace manual or manufacturer website to interpret codes. Error codes point directly to sensors, switches, or circuit issues.

Step 10: When To Replace Components Or Call A Professional

Replace igniters, flame sensors, and filters as first-line parts if diagnostics point to them. For gas valve, control board, heat exchanger, or complex venting problems, call a licensed HVAC technician. Document observed symptoms and error codes to expedite diagnosis.

Maintenance Tips To Prevent Ignition Problems

• Replace air filters every 1–3 months during use.
• Schedule annual professional furnace tune-ups before the heating season.
• Keep vents and returns clear and ensure proper attic or crawlspace insulation to avoid drafts that disturb a pilot.
• Clean burners and flame sensors annually to maintain reliable ignition.
• Monitor carbon monoxide detectors and install them near sleeping areas for safety.

When The Furnace Won’t Ignite — Professional Diagnostics

A professional will perform a series of tests: gas pressure, flame rod current, limit switch functionality, inducer and pressure switch operation, and control board diagnostics. These tests determine whether the issue is electrical, mechanical, or gas-related.

Professionals will also inspect the heat exchanger for cracks, a critical safety check when ignition or combustion seems inconsistent. Heat exchanger issues require immediate attention and may necessitate furnace replacement.

Cost Considerations And Typical Repairs

Common repair costs vary: flame sensor replacement is typically low-cost, igniters moderate, and gas valve or control board replacements higher. Professional diagnosis often costs a service call fee, with repair estimates provided after testing.

Repair	Typical Cost Range (U.S.)
Flame Sensor Replacement	$80 – $200
Igniter Replacement	$100 – $350
Thermocouple/Pilot Repair	$75 – $200
Gas Valve Replacement	$400 – $900
Control Board Replacement	$250 – $800

Key Safety Warnings

Never attempt to modify or override safety devices such as limit switches, flame sensors, or pressure switches. Tampering with gas valves or the heat exchanger can create life-threatening carbon monoxide or fire hazards.

If there is any suspicion of a cracked heat exchanger, persistent gas smell, or repeated lockouts after basic fixes, cease use and contact a licensed HVAC technician immediately.

SEO Keywords And Search Relevance

This article targets primary search phrases like “furnace won’t ignite”, “furnace shuts off”, and related queries such as “furnace lights then goes out”, “why furnace keeps shutting off”, and “flame sensor cleaning”. It provides diagnostic steps and clear next actions, matching typical user intent from troubleshooting searches.

Further Resources

Refer to the furnace owner’s manual, manufacturer support pages, and certified HVAC service providers for model-specific guidance and parts. For gas-related emergencies, contact the local gas utility or emergency services.

For more detailed diagnostics, homeowners can document error code blink sequences and video the ignition sequence to share with a technician for faster resolution.