Why Does My Furnace Pilot Light Keep Going Out

The pilot light or electronic ignition on a furnace that lights and then goes out is a common household problem that can indicate several underlying issues. This article explains the most likely causes, diagnostic steps, and practical solutions to help homeowners identify whether the issue is simple to fix or requires a qualified HVAC technician. Safety tips and preventive maintenance measures are included to reduce future occurrences.

Symptom	Possible Causes	Typical Fix
Pilot light lights then goes out	Dirty pilot orifice, draft, faulty thermocouple	Clean or replace parts, secure pilot shield, test thermocouple
Ignition sparks but burner won’t stay lit	Faulty flame sensor, gas valve issue, limit switch	Clean flame sensor, check gas pressure, replace valve or switch
Intermittent heating cycles	Overheating, clogged filter, blocked vent	Replace filter, clear vent, inspect fan and limit switch

How A Furnace Ignition System Works

Understanding the components helps pinpoint why a flame fails. Traditional gas furnaces use a pilot light or an electronic ignition (hot surface igniter or spark) to light the main burner. A pilot-based system has a pilot orifice, thermocouple, gas valve, and pilot assembly. Electronic systems add flame sensors, igniters, and control boards. When the thermostat calls for heat, the ignition lights gas and the flame sensor confirms combustion. If the control does not receive a proper signal, it will shut the gas supply for safety, causing the flame to go out.

Common Causes When Pilot Or Burner Lights Then Goes Out

Pilot Orifice Or Burner Port Blockage

Dust, spider webs, and airborne debris can clog the pilot orifice or burner ports, producing a weak flame that cannot sustain combustion. This is common in homes with infrequent furnace use or where the furnace is located in a dusty basement. Cleaning the orifice and burner often restores consistent flame.

Thermocouple Failure (Pilot Systems)

The thermocouple is a safety sensor that detects the pilot flame and permits gas to the main burner. If the thermocouple is dirty, bent out of position, or worn out, it may not register heat reliably and will shut off the gas. Symptoms include a pilot that lights but then goes out after a few seconds. Testing or replacing the thermocouple resolves this in many cases.

Faulty Flame Sensor (Electronic Ignition Systems)

Electronic furnaces use a flame sensor rather than a thermocouple. A flame sensor that is coated with soot or oxidation can fail to detect the flame. The control board then shuts off gas for safety. This usually occurs after the furnace runs for a few seconds and then trips off. Cleaning the flame sensor with fine emery cloth can restore proper sensing.

Drafts Or Airflow Issues

Strong drafts near the furnace cabinet or a blocked combustion air intake can blow out a weak pilot flame or disrupt burner stability. Drafts often come from open doors, windows, attic hatches, or negative pressure caused by exhaust fans. Sealing drafts and ensuring proper combustion air supply help stabilize the flame.

Gas Supply Problems

Intermittent or low gas pressure will cause the pilot or main burner to go out. This might be due to a partially closed shutoff valve, an issue with the gas meter, or supply problems from the utility. If multiple gas appliances are affected or there is a smell of gas, contact the utility or a technician immediately. Do not attempt complex gas repairs without proper credentials.

Control Board, Limit Switch, Or Safety Device Trip

Modern furnaces include multiple safety switches such as high-limit switches, rollout switches, and pressure switches. If a safety device opens, it interrupts gas flow or the ignition sequence causing the flame to go out. These trips are often caused by overheating, blocked vents, a failing inducer motor, or cracked heat exchangers. Diagnosing which switch tripped requires testing by a technician.

Faulty Gas Valve Or Ignition Module

A malfunctioning gas valve may fail to remain open after initial ignition, or an ignition module might not sustain the spark or hot surface heat. Such components can degrade over time and require replacement when they fail to hold consistent operation. Symptoms include repeated ignition attempt cycles with no stable flame.

How To Diagnose The Problem Safely

Working with gas appliances involves risk. Homeowners can perform basic noninvasive checks safely; anything involving gas connections, internal control components, or complex electrical diagnostics should be left to a licensed HVAC technician.

Initial Safety Checks

• Ensure the furnace switch and gas valve are ON and the thermostat is set above room temperature.
• Smell for gas before approaching. If there is a strong odor, evacuate and call the gas company or 911.
• Turn off power to the furnace at the switch or breaker before inspecting internal parts.

Simple Tests To Perform

1. Inspect The Pilot Or Burner Flame: A proper pilot flame should be steady and blue. Yellow, weak, or flickering flames indicate a problem.
2. Check For Drafts: Close nearby windows and doors, disable strong exhaust fans, and observe whether the flame stabilizes.
3. Replace Or Clean The Air Filter: A severely clogged filter can cause overheating and safety trips that interrupt ignition cycles.
4. Look For Error Codes: Many furnaces flash LED codes on the control board or display. Note the pattern and consult the manual.

Step-By-Step Fixes For Common Problems

Cleaning The Pilot And Burner

Turn off gas and power, then use compressed air or a thin wire to clear the pilot and burner ports. After cleaning, relight according to the furnace instructions. If uncomfortable doing this, schedule a service visit.

Replacing The Thermocouple

For pilot-equipped units, remove the thermocouple and inspect for dirt or bends. Replace with the correct length and type per the furnace manual. A properly positioned thermocouple tip should sit in the pilot flame. Tighten the connection to the gas valve to avoid leaks.

Cleaning The Flame Sensor

For electronic ignition units, remove the flame sensor, gently clean the metal rod with fine emery cloth or steel wool, then reinstall. Do not use sandpaper or aggressive abrasives. After cleaning, restart the furnace and monitor for stable operation.

Addressing Drafts And Combustion Air

Seal obvious drafts and verify that the furnace has adequate combustion air. Ensure the intake vent and flue are not blocked by debris, nests, or snow. Installing a proper vent hood or relocating exhaust fans can help maintain stable combustion.

Checking Gas Supply And Valves

Verify that the main gas shutoff is fully open and other gas appliances function normally. If the homeowner suspects supply issues, contact the gas utility before further troubleshooting. Technicians can measure gas pressure and inspect valves for internal faults.

Resetting Or Testing Safety Switches

Resettable switches may be safely reset after power cycling, but persistent trips indicate an underlying fault. Technicians use multimeters to test switch continuity and diagnose causes such as restricted airflow, fan failure, or heat exchanger issues.

When To Call A Professional

If the pilot or burner continues to go out after basic cleaning and simple checks, or if symptoms include gas odors, persistent error codes, or repeated safety switch trips, a licensed HVAC technician should be contacted. Qualified technicians have the tools to test gas pressure, inspect combustion, verify control board logic, and replace complex components safely.

Costs And Typical Repairs

Repair costs vary with parts and labor. Simple cleaning or flame sensor replacement is relatively inexpensive, often under a few hundred dollars. Replacing a gas valve, control board, or performing heat exchanger repairs can run into higher costs. Obtaining multiple estimates and checking technician credentials is recommended.

Preventive Maintenance To Prevent Reoccurrence

• Schedule Annual Furnace Tune-Ups: Professional inspections before heating season reduce failure risk.
• Change Filters Regularly: Replace filters every 1–3 months depending on usage and filter type.
• Keep The Area Clean: Prevent dust and pest nests near the pilot and burners.
• Seal Drafts And Ensure Proper Venting: Maintain clear intake and exhaust paths to avoid combustion instability.

Safety Reminders

Never ignore persistent ignition failures or gas odors. Do not attempt complex repairs on the gas system unless certified. Always shut off gas and power before opening the furnace, and use a licensed professional for gas or electrical component replacements.

Key Troubleshooting Checklist

Step	Action	Expected Outcome
1	Observe Flame	Blue stable flame or note flicker/yellow
2	Clean Pilot/Burner Or Sensor	Flame stabilizes if buildup was the issue
3	Check For Drafts/Blocked Vents	Less fluctuation when drafts removed
4	Verify Gas Supply	Other appliances work; no sudden pressure drop
5	Call Technician	Replace faulty valves, switches, or heat exchanger

Additional Resources

Homeowners can refer to the furnace manufacturer’s manual for model-specific error codes and component locations. Heating industry organizations and local utility websites provide safety guides and qualified contractor directories. When in doubt, prioritize safety and contact a licensed HVAC professional for diagnosis and repair.