Pilot Light Blows Out When Furnace Kicks On: Causes, Troubleshooting, and Fixes

The pilot light blowing out when the furnace kicks on is a common and potentially hazardous issue that can interrupt heating and signal underlying problems. This article explains likely causes, step-by-step troubleshooting, safety precautions, repair options, and preventative maintenance to help homeowners diagnose and address the problem effectively. Priority: safety first; if there is any gas smell or repeated failures, turn the gas off and call a professional.

Symptom	Possible Cause	Typical Fix
Pilot blows out when burner ignites	Faulty thermocouple, strong draft, or gas pressure spike	Replace thermocouple, seal drafts, adjust gas pressure
Pilot goes out intermittently	Dirty pilot orifice, weak gas supply, or vibration	Clean orifice, check gas line, tighten fittings
Smell of gas or clicking	Valve leak or ignition problem	Shut off gas, ventilate, call licensed technician

How Pilot Light Systems Work

Understanding the pilot light function helps narrow down causes when the pilot light blows out as the furnace starts. The pilot is a small, continuous flame that ignites the main burner when the thermostat calls for heat.

A thermocouple or flame sensor detects the pilot flame and keeps the gas valve open. When the main burner is commanded to light, the gas system shifts from pilot-only flow to main burner flow, and the pilot must remain lit through this transition.

Key components: pilot assembly (orifice and tube), thermocouple/flame sensor, gas control valve, burner and manifold, and venting/chimney system.

Common Causes Why The Pilot Light Blows Out When Furnace Kicks On

Pilot Orifice Blockage Or Dirty Pilot

A blocked or partially obstructed pilot orifice produces a weak flame that can be extinguished when the draft or gas flow changes as the burner engages. Soot, dust, or spider webs inside the pilot tube are frequent culprits.

Faulty Or Failing Thermocouple Or Flame Sensor

The thermocouple generates a small voltage while heated by the pilot. If it is worn, bent, or mispositioned, it may fail to detect flame reliably during the transient that occurs when the main burner starts, causing safety shutoff and extinguishing the pilot.

Improper Gas Pressure Or Supply Issues

When the main burner opens, a pressure drop or surge in the gas line can momentarily affect the pilot flame. Low supply pressure, clogged regulator, or competing appliances on the same line can cause the pilot to sputter or go out.

Strong Drafts Or Venting Problems

A draft in the furnace cabinet, flue, or nearby ductwork can blow the pilot out when the main burner changes the airflow pattern. Negative pressure caused by improper venting or exhaust fan operation can interfere with pilot stability.

Faulty Gas Control Valve Or Pilot Tube Leak

A malfunctioning gas valve may not proportion gas correctly between pilot and main burner during ignition. Leaks or loose fittings in the pilot tube can reduce pilot flame strength and lead to extinction at startup.

Vibration Or Mechanical Disturbance

Loose mounting, vibration from an unbalanced blower, or a sticking gas valve can momentarily disrupt the pilot flame as the furnace transitions to full operation.

Step-By-Step Troubleshooting Guide

Before starting any troubleshooting, confirm there is no strong odor of gas. If gas smell exists, evacuate the home, shut off the gas at the meter if safe, ventilate, and call the gas company or a licensed technician immediately.

Visual Inspection

Inspect the pilot assembly for visible soot, corrosion, or misalignment. Check the thermocouple or flame sensor for discoloration or deposits. Look for loose wires and fittings and observe venting for obstructions.

Verify Pilot Flame Shape And Position

A healthy pilot flame is steady, blue, and wraps the thermocouple. A yellow, flickering, or elongated flame indicates poor combustion, blockages, or incorrect gas pressure.

Test Thermocouple Or Flame Sensor

While the pilot is lit, the thermocouple should produce millivolts; a weak thermocouple often fails when the main burner starts. For furnaces with an electronic flame sensor, carefully clean the sensor rod with fine steel wool or emery cloth to remove oxidation, then test operation.

Check For Drafts And Venting Issues

Temporarily block nearby openings or run the furnace with a door ajar to see if drafts influence pilot stability. Inspect chimney or vent connectors for soot, animal nests, or displacement that could alter airflow and extinguish the pilot.

Observe Gas Valve Behavior

Listen for unusual noises at the gas control valve during ignition. If the pilot goes out only when the valve shifts to main burner mode, the valve may be defective or sticking. Do not attempt internal valve repairs; contact a licensed technician for valve testing or replacement.

Check Gas Pressure And Supply

If multiple appliances are affected or the pilot weakens when other gas devices start, have a qualified technician check regulator and line pressure. Pressure testing requires specialized gauges and should be performed by a professional.

DIY Fixes Homeowners Can Safely Try

Clean The Pilot And Flame Sensor

Turning the gas off and allowing the pilot assembly to cool, use compressed air and a soft brush to remove debris. For the flame sensor, gently polish the metal rod until bright, then reassemble. Cleaning often restores reliable sensing and pilot stability.

Reposition Or Replace Thermocouple

If the thermocouple tip is not fully enveloped by the pilot flame, loosen the mounting, carefully reposition the tip into the flame, and re-tighten. If repositioning does not help, replace the thermocouple; the part is inexpensive and commonly available.

Tighten Fittings And Check For Drafts

Inspect and tighten the pilot tube fittings and nearby gas connections. Seal gaps in the furnace cabinet or around vents that may produce drafts. Simple sealing can reduce airflow disruptions that blow out the pilot.

Inspect And Replace Pilot Orifice If Clogged

If cleaning does not restore a steady flame, remove the pilot orifice for inspection. Use a thin wire or orifice cleaning tool to clear blockages. Replace the orifice if damaged. Exercise caution—work on gas appliances only when comfortable with the procedure and with gas shut off.

When To Call A Professional

Homeowners should call a licensed HVAC technician or gas plumber if any of the following occur: persistent pilot outages despite cleaning and thermocouple replacement, gas odor, suspected gas valve malfunction, pressure issues, complex venting problems, or uncertainty about safety procedures.

Reasons to hire a pro: gas control valve replacement, regulator or pressure testing, chimney and vent repairs, or when code compliance and permits are required.

Typical Repair Options And Cost Estimates

Costs vary by region and complexity; these ranges are approximate for U.S. homeowners and exclude taxes or service fees.

Repair	Typical Cost Range
Thermocouple Replacement	$50–$150 (parts and labor)
Flame Sensor Cleaning/Replacement	$75–$200
Pilot Orifice Cleaning/Replacement	$50–$150
Gas Control Valve Replacement	$300–$800+
Venting/Flue Repair	$150–$1,000+ depending on scope

Smaller parts like thermocouples are inexpensive for DIY, but labor and safety considerations often make professional service the prudent choice.

Safety Precautions And Best Practices

Always prioritize safety when dealing with gas appliances. If the smell of gas is present, leave the area immediately and contact your gas provider or emergency services.

Turn off the gas supply before cleaning or disassembling components, and relight pilots per manufacturer instructions. Use tools appropriate for gas fittings and avoid makeshift repairs to valves or connectors.

Schedule annual HVAC inspections to catch pilot and combustion issues before they cause outages or carbon monoxide risks. Install carbon monoxide detectors near sleeping areas and the furnace room for added safety.

Preventive Maintenance To Keep The Pilot Stable

• Annual professional tune-ups including pilot, burners, and gas valve inspection.
• Regularly clean the furnace cabinet and replace filters to reduce dust and debris near the pilot.
• Seal gaps and control drafts around the furnace area and attic access.
• Monitor other gas appliances for symptoms of low pressure or shared-supply issues.
• Replace aging components proactively, especially thermocouples and flame sensors, as part of scheduled maintenance.

Frequently Asked Questions

Can a bad thermocouple be the only reason the pilot goes out when the furnace starts?

Yes, a failing thermocouple is a common and likely cause because it may not generate sufficient voltage during the transient when the main burner opens, causing the gas valve to shut the pilot off as a safety response.

Is it safe to relight the pilot myself?

Relighting the pilot is safe if no gas odor is present, the homeowner follows the furnace manual, and the gas supply and controls appear normal. If the pilot goes out repeatedly, stop relighting and call a professional.

Could a chimney or vent blockage cause this issue?

Yes. Blocked vents, animal nests, or damaged flue pipes can change airflow dynamics and create drafts that extinguish the pilot when the main burner starts.

How long does it typically take a technician to fix this problem?

Simple fixes like cleaning or thermocouple replacement often take under an hour. Complex issues like gas valve replacement, venting repair, or pressure troubleshooting may require several hours and possibly follow-up visits.

Final Practical Checklist For Homeowners

• Smell gas? Evacuate and call for emergency help.
• Inspect pilot flame shape—should be steady and blue.
• Clean pilot and flame sensor if comfortable doing so.
• Check and reposition thermocouple; replace if necessary.
• Look for drafts, loose fittings, or vent obstructions.
• If problem persists, contact a licensed HVAC/gas professional for diagnosis and repair.

Addressing a pilot light that blows out when the furnace kicks on requires a mix of safety awareness, targeted troubleshooting, and timely repairs. By following the steps above, homeowners can often identify the root cause and decide whether a DIY fix or professional service is appropriate.