Gas Furnace Lights Then Shuts Off: Causes, Diagnostics, and Fixes

The article explains why a gas furnace lights then shuts off, the most common causes, step-by-step diagnostics, and safe fixes homeowners or technicians can use. It covers ignition systems, airflow, safety switches, and when to call a professional, focusing on practical troubleshooting for U.S. households.

Symptom	Likely Causes	Suggested Action
Furnace Lights Then Shuts Off Immediately	Flame Sensor, Ignition Malfunction, Clogged Air Filter	Clean flame sensor, check error codes, replace filter
Furnace Runs Briefly, Then Stops	Overheating, Blower Failure, Pressure Switch Issues	Inspect limit switch, check blower motor, verify vent clearance
Intermittent Starts And Stops	Thermostat Wiring, Gas Valve Problems, Draft Issues	Test thermostat, check gas supply, examine venting

How A Gas Furnace Starts And Why It Might Shut Off

A gas furnace follows a sequence: the thermostat calls for heat, the inducer motor and safety checks run, the ignition system lights the burners, the flame is sensed, and the blower distributes warm air. If any component fails the safety checks, the furnace will light then shut off to prevent unsafe operation. Understanding this sequence helps isolate where the interruption occurs.

Common Causes Of A Furnace Lighting Then Shutting Off

Dirty Or Faulty Flame Sensor

The flame sensor confirms the burner flame is present. A contaminated or failing flame sensor often allows ignition but then the control board shuts the gas valve because it does not detect flame. Cleaning or replacing the sensor is a common and effective fix.

Ignition System Problems

Modern furnaces use hot surface igniters or intermittent pilot spark igniters. A weak igniter may light briefly before failing, causing the control board to shut off the gas. Test the igniter visually and electrically and replace if cracked or slow to heat.

Airflow Restrictions And Overheating

A clogged air filter, blocked return or supply vents, or a malfunctioning blower can cause the heat exchanger to overheat. The high-limit switch turns the burner off to protect the furnace. Check and replace filters, clear vents, and inspect the blower.

Pressure Switch Or Venting Issues

Exhaust vent blockages, closed dampers, or a failing inducer motor can cause incorrect pressure readings. The pressure switch prevents the furnace from running if the venting system is compromised, often causing a light-then-shut cycle.

Gas Valve Or Supply Problems

A partial gas supply or a malfunctioning gas valve may allow ignition but not sustain a consistent flame. Verify the gas supply line and pilot or electronic valve operation. Only qualified technicians should service gas valves.

Thermostat And Control Board Faults

Faulty thermostat signals or control board errors can cause intermittent operation. Loose wiring or bad relays may trigger a start followed by an immediate shutdown. Inspect wiring connections and look for error codes on the furnace board.

How To Diagnose A Furnace That Lights Then Shuts Off

Safety First

Turn off power to the furnace at the breaker before opening access panels. If a gas smell is present, evacuate the home and call the gas company or emergency services. For gas valve or complex control work, contact a licensed HVAC technician.

Check Error Codes And Status Lights

Most furnaces have LEDs or digital displays that flash error codes. Record the pattern and consult the furnace manual or manufacturer website. Error codes often point directly to pressure switch, flame sense, or limit switch problems.

Inspect And Clean The Flame Sensor

Remove the flame sensor, gently clean it with fine-grit emery cloth or steel wool to remove oxidation, then reinstall. If cleaning does not fix the issue, replace the sensor. This is a common DIY fix for the light-then-shut symptom.

Test The Igniter And Ignition Sequence

Observe the ignition: a hot surface igniter should glow bright orange; a spark igniter should produce consistent sparks. If the igniter fails or is slow, replace it. Measure resistance with a multimeter if comfortable doing so.

Evaluate Airflow And The Blower

Check air filters, return grilles, and supply vents for restriction. Inspect the blower wheel and motor; listen for unusual noises or brief starts and stops. Replace clogged filters and clear obstructions to improve airflow and avoid overheating.

Verify Venting And Inducer Operation

Inspect the exhaust and fresh-air intake for blockages such as bird nests, debris, or snow. Confirm the inducer motor runs smoothly during startup. A stuck or weak inducer can cause the pressure switch to open and shut the burner off.

Check Gas Supply And Valve Function

Confirm the gas valve is open at the service line and that other gas appliances operate. Do not attempt internal gas valve repairs. If gas flow seems restricted, consult the gas utility or a licensed technician.

Examine Thermostat And Wiring

Replace batteries in wireless thermostats and ensure proper wiring and voltage. A malfunctioning thermostat can create brief calls for heat that stop unexpectedly. Temporarily set the furnace to “on” and test direct control at the furnace to isolate the thermostat.

DIY Fixes Homeowners Can Try

• Clean or Replace The Flame Sensor: Turn off power, remove and clean the sensor, then test.
• Replace The Air Filter: Use the correct size and MERV rating for the system.
• Clear Vents And Intake: Remove debris from exterior vent terminations.
• Reset The Furnace: Power-cycle the unit to clear temporary faults and observe startup behavior.
• Check Thermostat Settings: Ensure proper temperature differential and stable wiring connections.

When To Call A Professional HVAC Technician

Contact a licensed technician if cleaning and basic checks do not resolve the issue, if a gas smell is present, or if the problem involves gas valves, pressure switches, control boards, or refrigerant-related components. Technicians have specialized tools for gas pressure, flame diagnostics, and electrical testing.

Parts That Often Need Replacement

Common replacement parts for the light-then-shut symptom include flame sensors, igniters, pressure switches, limit switches, inducer motors, and control boards. Costs and complexity vary; request a written estimate and ensure the technician uses OEM or compatible parts.

Preventive Maintenance To Avoid Future Shutdowns

• Schedule annual professional furnace tune-ups before heating season.
• Replace air filters every 1–3 months depending on filter type and household conditions.
• Keep furnace area and vents clear of obstructions and lint.
• Inspect exterior vent terminations seasonally for nests and debris.
• Install carbon monoxide detectors near sleeping areas and the furnace room.

Safety And Carbon Monoxide Considerations

A furnace that cycles irregularly can risk incomplete combustion. Install and test carbon monoxide detectors and replace batteries yearly. If CO detector alarms or occupants feel symptoms like headache or nausea, evacuate and call emergency services and the gas company immediately.

Cost Expectations For Repairs

Minor repairs like cleaning a flame sensor or replacing a filter are low cost. Typical parts and labor for igniter or sensor replacement range from a modest service call to a few hundred dollars. Complex issues like a failed control board, inducer motor, or gas valve are higher cost. Always get multiple quotes for major repairs.

Key Diagnostic Checklist For Technicians

1. Read furnace error codes and document LED patterns.
2. Confirm proper gas pressure and valve operation.
3. Inspect and test flame sensor and igniter.
4. Verify inducer motor and pressure switch operation.
5. Check limit switches and blower function for overheating signatures.
6. Examine venting, condensate lines (if high-efficiency unit), and intake air supply.
7. Test thermostat signals and control board relays.

Frequently Asked Questions

Why Does The Furnace Light Then Go Out After A Few Seconds?

Most often due to a dirty flame sensor, ignition failure, or safety switch tripping from improper venting or overheating. The control board shuts the gas to prevent unsafe operation.

Can Homeowners Fix This Without A Technician?

Homeowners can safely replace filters, clean the flame sensor, clear vents, and reset the furnace. For gas valve, pressure switch, or control board issues, a licensed HVAC professional is recommended.

Is It Dangerous If The Furnace Keeps Cycling?

Continuous short cycling reduces efficiency, increases wear, and can create safety risks including incomplete combustion. Prompt diagnosis and repair are important, especially if accompanied by unusual odors or CO alarm activation.

Useful Resources And Manufacturer References

Consult the furnace’s owner manual for model-specific error codes and procedures. Manufacturer websites (e.g., Carrier, Trane, Lennox, Goodman) provide troubleshooting guides. Local utility companies and certified HVAC associations can recommend licensed contractors and safety advice.

Final Practical Tips

Keep a simple maintenance kit: replacement filter, soft brush or emery cloth for flame sensor cleaning, and the furnace manual. Document symptoms, error codes, and the sequence of events to help technicians diagnose faster. Regular maintenance prevents many causes of a gas furnace that lights then shuts off.