Furnace Ignites but Won’t Stay On: Causes and Fixes

The furnace ignites but won’t stay on, leaving a home cold and a homeowner puzzled. This article explains common causes, diagnostic steps, safety considerations, and repair options to help identify why a furnace lights briefly then shuts off. It focuses on gas furnaces typical in U.S. homes and gives practical guidance for homeowners and technicians.

Symptom	Possible Cause	Typical Fix
Ignites then shuts off	Faulty flame sensor	Clean or replace sensor
Short burners run	Dirty burners or ignition	Service burners and ignition system
Blower runs but flame goes out	Gas pressure or valve issue	Check gas supply; replace valve
Cycles rapidly	Thermostat or limit switch	Calibrate or replace
No steady ignition	Control board or wiring	Diagnose control board; repair wiring

How A Typical Gas Furnace Ignition Cycle Works

Understanding the ignition cycle helps narrow causes when a furnace ignites but won’t stay on. A thermostat call for heat triggers the control board, which activates the inducer motor, opens the pressure switch, and energizes the ignition source. After a flame is established, a flame sensor confirms combustion and the gas valve remains open while the blower circulates air.

If any component in this chain fails, the control board will shut fuel flow for safety, causing the furnace to light briefly then stop.

Common Causes When A Furnace Lights Then Shuts Off

Flame Sensor Problems

The flame sensor detects flame presence by measuring a small current. A weak or dirty sensor can fail to sense flame even when burners are lit, prompting the control board to shut off the gas valve. This is one of the most frequent reasons a furnace lights but doesn’t stay on.

Signs include brief ignition followed by multiple restart attempts and error codes indicating flame failure.

Blocked Or Dirty Burners And Ignition Components

Soot, dust, or debris on burners, ignitors, or pilot assemblies can cause intermittent flame or weak combustion. A weak flame may not make solid contact with the sensor, or combustion may be unstable, causing the furnace to shut off.

Faulty Gas Valve Or Low Gas Pressure

If the gas valve is failing or incoming gas pressure is insufficient, the burners may light briefly then fail to maintain full flame. Utility supply issues or partially closed shutoff valves can produce similar behavior.

Pressure Switch Or Inducer Motor Issues

The inducer motor and pressure switch ensure safe venting before ignition. A failing inducer motor, blocked vent, or a pressure switch that doesn’t stay closed will interrupt the cycle after ignition, causing the furnace to stop.

Limit Switch And Overheat Protection

High-limit switches shut the furnace down if heat exchangers or plenum temperatures exceed safe thresholds. Restricted airflow from clogged filters or closed vents can cause rapid heat buildup and trigger the limit switch shortly after ignition.

Thermostat And Control Board Faults

Faulty thermostat signals or a malfunctioning control board can cut the call for heat prematurely. Wiring issues, short circuits, or failed relays on the control board may allow ignition but then remove the command to keep the gas valve open.

Ignitor Or Pilot Problems

In electronic ignition systems, a weak or failing hot surface ignitor or intermittent spark can cause unstable ignition. With intermittent ignition, the control board may attempt several ignitions before locking out the system.

Safety First: What To Do Immediately

If the furnace cycles rapidly, emits unusual odors, or if the homeowner suspects a gas leak, shut off the gas supply at the main valve and contact the gas company and a qualified technician. Do not attempt complex repairs without proper training.

For non-emergency intermittent shutdowns, turn off the furnace and inspect accessible components like the filter and visible wiring before calling a professional.

Step-By-Step Diagnostic Checklist

1. Check Air Filter And Vents

A clogged filter restricts airflow and can trip the high-limit switch. Replace disposable filters monthly during heavy use or as recommended, and clean washable filters. Ensure vents and returns are open and unobstructed.

2. Observe The Ignition Sequence

On a working system the inducer should start, pressure switch should close, ignition should occur, and the flame sensor should confirm flame. Note at what point the furnace stops: after ignition, during blower start, or immediately after flame appears.

3. Inspect And Clean The Flame Sensor

Turn off power and gas. Remove the flame sensor and gently clean the metal rod with fine grit pad or steel wool to remove oxidation and soot. Reinstall and test. Cleaning often restores reliable sensing.

4. Check Burners And Ignitor

Inspect burners for rust, debris, or misalignment. For hot surface ignitors, look for cracks. Replace ignitors that show damage. Clean burner ports carefully to restore even flames.

5. Verify Gas Supply And Pressure

Ensure the household gas valve is fully open. If multiple appliances show low flame or performance issues, contact the gas utility. Technicians can measure manifold pressure and test the gas valve.

6. Test The Pressure Switch And Inducer

Check for proper inducer operation and listen for unusual noises. Inspect vent pipes for blockages or nesting. Technicians can test pressure switch continuity and tubing for leaks.

7. Inspect Limit Switch And Blower Operation

Make sure the blower runs at the appropriate time. A malfunctioning blower or motor that runs late can cause overheating and rapid shutdown. Clean the blower assembly and motor vents if dusty.

8. Check Control Board And Thermostat Signals

Examine error LED codes on the control board and consult the furnace manual for interpretation. Verify thermostat wiring and settings. Replace the control board or thermostat only after confirming wiring and signals cause the issue.

When To Call A Professional

A homeowner can perform basic checks like replacing filters and cleaning a flame sensor, but call a licensed HVAC technician when the issue involves gas components, pressure testing, control board replacement, or if cleaning does not resolve the problem.

Call a pro if: gas smell is present, flame patterns are uneven, inducer motor sounds abnormal, pressure switch or gas valve testing is required, or error codes persist after basic maintenance.

Common Repairs And Typical Costs

Repair	Typical Cost Range (U.S.)	Notes
Clean Flame Sensor	$60–$150	Often quick; low parts cost
Replace Flame Sensor	$120–$300	Part cost varies by model
Clean/Service Burners	$100–$300	Depends on access and condition
Replace Ignitor	$150–$400	Hot surface ignitors are fragile
Gas Valve Replacement	$300–$700+	Skilled labor and testing required
Control Board Replacement	$250–$900+	Model-dependent; includes programming

Preventive Maintenance To Keep Furnaces Running

Regular maintenance prevents many ignition and short-run issues. Schedule annual professional tune-ups and perform seasonal checks like filter changes every 1–3 months.

• Change Filters Regularly: Improves airflow and reduces limit trips.
• Annual Inspection: Certified tech should inspect burners, heat exchanger, venting, and controls.
• Keep Vents Clear: Maintain return and supply registers and ensure attic/soffit vents are not blocked.
• Monitor Ignition Behavior: Note repeated short cycles and address promptly to avoid system stress.

Troubleshooting Grid: Quick Cause-Effect Checks

Symptom	Quick Check	Likely Fix
Furnace lights then dies after a few seconds	Clean flame sensor; observe flame	Clean/replace sensor
Rapid cycling on and off	Check filter and blower operation	Replace filter; clean blower
Igniter glows but no sustained flame	Inspect gas valve and gas supply	Test/replace gas valve or call utility
Inducer runs then stops after ignition	Check venting and pressure switch tubing	Clear vent; replace pressure switch

When Component Replacement Is Necessary

Replacement is justified when cleaning or adjustment fails, when components show visible damage, or when diagnostics show electrical faults. Consider age of the furnace: older units nearing 15–20 years may be better replaced than repeatedly repaired.

Signs replacement may be better: frequent breakdowns, poor efficiency, cracked heat exchanger, or obsolete controls hard to source.

Energy And Safety Considerations

Frequent short cycling reduces efficiency and increases wear, raising energy costs and the chance of component failure. A malfunctioning furnace can also pose safety risks such as incomplete combustion and carbon monoxide production. Ensure carbon monoxide detectors are installed and operational.

Summary: Practical Next Steps

• Replace the air filter immediately if dirty.
• Clean the flame sensor as a low-cost first step.
• Observe the ignition sequence to identify when shutdown occurs.
• Contact a licensed HVAC technician for gas valve, pressure, or control board issues.
• Schedule annual preventive maintenance to prevent recurrence.

Addressing a furnace that ignites but won’t stay on usually begins with simple maintenance and progresses to professional diagnostics when necessary. Timely action improves safety, restores reliability, and extends furnace life.