A gas furnace that ignites then goes out can leave a home cold and a homeowner frustrated. This article explains the common causes, step-by-step troubleshooting, and safe repair options for furnaces that start but fail to stay lit. It focuses on practical diagnostics and clear actions to take, helping readers understand when a simple fix is possible and when to call a professional.
| Symptom | Likely Cause | Quick Fix |
|---|---|---|
| Burner Ignites Then Immediately Dies | Flame Sensor Dirty, Faulty Gas Valve, Or Pressure Switch | Clean Sensor, Test Switch, Call Technician |
| Ignites, Runs Briefly, Then Shuts Off | Flame Loss Due To Draft Or Venting Issue | Check Venting, Blockages, Ensure Proper Combustion Air |
| Pilot Lights Then Goes Out (Older Furnaces) | Thermocouple Or Pilot Orifice Problem | Replace Thermocouple/Pilot Orifice |
How A Gas Furnace Ignition System Works
Understanding the ignition sequence helps narrow down why a furnace ignites then goes out. Modern furnaces use an electronic ignition or hot surface igniter that lights the burners when the thermostat calls for heat. A flame sensor confirms flame presence and keeps the gas valve open. Various safety switches — including the pressure switch, limit switch, and rollout switch — monitor safe operation and shut the furnace down if a fault is detected.
Common Causes Why A Gas Furnace Ignites Then Goes Out
Dirty Or Faulty Flame Sensor
The flame sensor detects that burners are lit. If it becomes coated with carbon or residue, it may fail to sense the flame and will shut the gas valve to prevent unburned gas accumulation. This is one of the most common reasons a furnace lights briefly then shuts off.
Pressure Switch Or Combustion Air Problems
The pressure switch ensures the inducer motor and venting are working so combustion gases flow correctly. If the switch fails or the vent intake/exhaust is blocked, the switch will open and the furnace will shut down shortly after ignition.
Thermostat Or Control Board Issues
A faulty thermostat or failing control board can terminate the call for heat early. Loose wiring, a misconfigured thermostat, or intermittent control board faults may cause the furnace to light then stop.
Gas Valve Or Gas Supply Interruptions
If the gas valve is failing or the supply pressure is inconsistent, burners may light but not sustain. Issues with the utility supply, a partially closed gas valve, or internal valve failure can cause temporary ignition followed by shutdown.
Dirty Burners Or Pilot Assembly (Older Models)
Accumulated dirt on burners or a clogged pilot orifice can lead to a weak flame that goes out. In older standing pilot furnaces, a failing thermocouple that cannot sense pilot flame will shut off gas flow to the pilot and main burners.
Limit Switch Or Rollout Switch Activation
Overheating or flame rollout triggers the limit or rollout switches. If airflow is restricted (dirty filter or blocked vents), heat builds fast and the limit switch may turn off the burners after they ignite.
Step-By-Step Troubleshooting
Safety First
Turn off power and gas before inspecting internal components. If a gas odor is present, leave the home immediately and call emergency services or the gas utility. Only perform noninvasive checks unless qualified.
Check The Thermostat Call For Heat
Confirm the thermostat is set to heat and a higher temperature than room temperature. Replace batteries if applicable and verify wiring connections at the thermostat and furnace control board are tight.
Inspect And Replace The Air Filter
A clogged filter causes restricted airflow and can trigger limit switch shutdowns. Replace the filter with the manufacturer’s recommended type and rating to restore proper airflow and prevent overheating.
Clean The Flame Sensor
To address a dirty flame sensor: turn off power and gas, remove the sensor (usually a thin metal rod in front of the burner), gently clean it with a fine abrasive pad or emery cloth, reassemble, restore power, and test. Cleaning often resolves the ignite-then-go-out symptom.
Inspect Venting And Combustion Air
Check for bird nests, debris, snow, or other blockages at intake and exhaust vents. Ensure exhaust vent termination is intact and inducer motor runs during ignition. Restricted venting can cause the pressure switch to remain open, cutting off burners.
Listen For The Inducer Motor And Pressure Switch Clips
Observe the ignition cycle: the inducer motor should start, the pressure switch should close (audible click), then ignition occurs. If the pressure switch never closes or opens quickly after ignition, the problem likely lies with venting, the inducer, or the switch itself.
Examine Burners And Pilot (Older Units)
Inspect burners for dirt, rust, or misalignment. For pilot-equipped furnaces, ensure the pilot flame envelops the thermocouple. Replace the thermocouple if the pilot extinguishes when slightly disturbed.
Test The Gas Valve And Gas Pressure
Gas valve diagnosis and manifold pressure checks should be performed by a licensed technician. Signs of a bad gas valve include clicking, inconsistent flame, or complete loss of gas flow after initial ignition.
When To Replace Parts Versus Call A Professional
Simple tasks like replacing a filter or cleaning a flame sensor can be done by a confident homeowner. Any work involving gas lines, gas valve replacement, control board diagnostics, or pressure tests should be handled by a licensed HVAC technician to ensure safety and code compliance.
Maintenance Tips To Prevent Future Ignition Failures
- Change filters regularly: Follow the furnace manufacturer’s schedule or every 1–3 months for standard filters.
- Annual professional tune-up: A certified technician can clean burners, test safety switches, and verify gas pressures.
- Keep vents clear: Maintain clear intake and exhaust terminations and check for pests seasonally.
- Inspect flame sensor annually: Cleaning during maintenance visits prevents corrosion and buildup.
- Seal duct leaks: Improve airflow and reduce limit-switch trips by sealing accessible ducts.
Estimated Costs For Repairs And Parts
Repair costs vary by region, furnace age, and part complexity. Typical ranges include: flame sensor replacement $80–$200 including labor, pressure switch replacement $150–$400, gas valve replacement $300–$1,000, and control board replacement $400–$1,200. A professional inspection fee may apply.
Advanced Diagnostics Professionals Use
Technicians use multimeters, combustion analyzers, manometers, and HVAC-specific diagnostic tools to test flame sensing voltage, gas manifold pressure, pressure switch differential, and control board error codes. These tests identify intermittent faults that are hard to detect through visual inspection alone.
Common Error Codes And What They Mean
Modern furnaces display LED flash codes or thermostat error messages. Examples include short flash patterns for flame failure sensing, long flashes for pressure switch faults, and continuous blink for limit switch trips. Refer to the furnace model’s technical sheet to decode the exact meaning.
Safety And Carbon Monoxide Considerations
Any intermittent ignition or incomplete combustion raises the risk of carbon monoxide (CO). Install CO detectors near sleeping areas and the furnace room. If a CO alarm triggers, evacuate and contact emergency services and a qualified HVAC professional. Regular maintenance reduces CO risks by ensuring proper combustion.
Frequently Asked Questions
Can A Dirty Flame Sensor Cause The Furnace To Run For A Short Time?
Yes. A dirty flame sensor often allows ignition but fails to detect steady flame, causing the control board to close the gas valve within seconds. Cleaning the sensor commonly fixes this issue.
Why Does The Furnace Start Working After Several Attempts?
Intermittent thermostat signals, weak pilot flame, or a gas valve struggling to stay open can cause repeated restarts. Repeated attempts may indicate failing components requiring professional diagnosis.
Is It Safe To Keep Restarting The Furnace When It Goes Out?
Repeatedly restarting a furnace that shuts off can mask a safety issue. It is safer to investigate the root cause, check for obvious problems (filter, vents, pilot), and call a technician if the cause is not clear.
Checklist For Homeowners Before Calling A Technician
- Replace the air filter and ensure proper airflow.
- Inspect intake and exhaust vents for visible blockages.
- Clean the flame sensor if comfortable doing so.
- Confirm thermostat settings and wiring are correct.
- Note LED error codes or sequence of events to report to the technician.
Summary Of Key Takeaways
Most common causes of a furnace that ignites then goes out include a dirty flame sensor, pressure switch or venting issues, failing gas valve, and thermostat or control board faults. Simple fixes like cleaning the flame sensor and replacing filters often resolve the issue. For gas-related components, pressure testing, and complex electrical diagnostics, hire a licensed HVAC technician to ensure safety and proper repair.
Following routine maintenance and addressing early warning signs helps prevent unexpected heating failures and extends the lifespan of the furnace system.