When your furnace pilot light won't stay lit, the issue typically stems from a few common components: a faulty or dirty thermocouple, a clogged pilot orifice, gas supply problems, or drafts affecting the flame. The thermocouple is the most frequent culprit—this safety sensor detects whether the pilot flame is burning and signals the gas valve to stay open. If it malfunctions or isn’t positioned correctly, the gas valve shuts off, extinguishing the pilot. Dirty pilot orifices or thermocouples can also weaken the flame, preventing it from generating enough heat to keep the thermocouple engaged. Less commonly, issues like low gas pressure, a faulty gas valve, or airflow disruptions (such as drafts or improper venting) may be to blame.

• Thermocouple problems are the leading cause, accounting for most pilot light failures ^[1][4][6]
• Dirty pilot orifices or thermocouples can create a weak, lazy flame that fails to heat the thermocouple sufficiently ^[2][5][8]
• Gas supply issues, including low pressure or regulator faults, may prevent consistent flame maintenance ^[4][5]
• Drafts or venting problems can blow out the pilot or disrupt combustion ^[3][8]

Before attempting repairs, always turn off the gas supply and follow safety protocols. If basic troubleshooting doesn’t resolve the issue, professional inspection is recommended—especially for older furnaces or complex gas valve problems.

Troubleshooting and Fixing a Furnace Pilot Light

Diagnosing the Thermocouple and Pilot Assembly

The thermocouple is a critical safety component that senses whether the pilot flame is active. When functioning correctly, it generates a small electrical current (typically 25-35 millivolts) that keeps the gas valve open. If the thermocouple is faulty, dirty, or improperly positioned, it won’t produce enough voltage, causing the gas valve to close and the pilot to extinguish. A weak or yellow-tipped pilot flame—rather than a sharp, blue flame—often indicates the thermocouple isn’t receiving adequate heat.

To diagnose the thermocouple:

• Inspect the flame: The pilot flame should be strong and blue, enveloping the top 1/2 inch of the thermocouple. A lazy, yellow, or flickering flame suggests dirt buildup or improper gas flow ^[2][6].
• Test voltage output: Use a multimeter to measure the thermocouple’s voltage while the pilot is lit. A reading below 25 millivolts confirms it needs replacement ^[6][10].
• Check positioning: The thermocouple tip must sit directly in the flame. If it’s bent or misaligned, adjust it so the flame fully engulfs the sensor ^[6].
• Clean the thermocouple: Use fine-grit sandpaper or steel wool to remove soot or corrosion from the tip. Avoid over-scrubbing, as this can damage the sensor ^[8].

If the thermocouple tests below 25 millivolts or cleaning doesn’t restore function, replacement is necessary. Thermocouples are inexpensive (typically $10–$20) and widely available at hardware stores ^[1]. When replacing, ensure the new thermocouple matches the furnace’s specifications, as incorrect types may not generate sufficient voltage.

Addressing Gas Flow and Draft Issues

Even with a functional thermocouple, the pilot light may fail if gas flow is inconsistent or external factors disrupt the flame. Common causes include clogged pilot orifices, gas regulator failures, or drafts from nearby vents or windows.

Gas flow problems:

• Dirty pilot orifice: The pilot orifice—a small opening where gas exits—can become clogged with dust or spider webs, restricting gas flow. Use a needle or compressed air to clear the orifice, but avoid enlarging the hole ^[2][5].
• Gas regulator issues: A faulty regulator may cause pressure fluctuations, leading to an unstable pilot flame. If other gas appliances (like stoves) also show weak flames, the regulator likely needs professional replacement ^[4][5].
• Low gas supply: Contact your gas provider to check for supply interruptions or pressure drops in your area ^[5].

Draft and venting issues:

• Airflow disruptions: Drafts from open windows, exhaust fans, or leaky ductwork can blow out the pilot. Seal gaps around the furnace and ensure vents are properly installed ^[3][8].
• Improper venting: Blocked or incorrectly sized vent pipes can cause backdrafting, where combustion gases (and flame) are pushed back into the furnace. Inspect vents for obstructions like debris or animal nests ^[8].
• Condensation buildup: In colder climates, condensation in vent pipes can drip onto the pilot, extinguishing it. Insulate vents or adjust their slope to prevent water accumulation ^[8].

For gas-related issues, exercise extreme caution. If you smell gas or suspect a leak, shut off the gas supply immediately, ventilate the area, and contact a professional. Never attempt to repair gas lines or regulators without proper training.

When to Call a Professional

While many pilot light issues can be resolved with basic troubleshooting, some scenarios require professional intervention. Consider contacting an HVAC technician if:

• The thermocouple and pilot assembly are clean and properly positioned, but the pilot still won’t stay lit ^[3][9].
• The furnace is over 15–20 years old, as older units may have failing gas valves or control modules that aren’t cost-effective to replace ^[10].
• You’re uncomfortable working with gas lines or electrical components. Modern furnaces often integrate safety features like limit switches or electronic ignition systems that require specialized diagnostics ^[1][7].
• The pilot lights but the main burner fails to ignite, indicating potential issues with the gas valve, control board, or flame sensor ^[7].

Professional inspections typically cost $80–$150, but they can identify underlying problems like cracked heat exchangers or carbon monoxide leaks—issues that pose serious safety risks if overlooked ^[4]. For older furnaces, technicians may recommend upgrading to a newer model with electronic ignition, which eliminates pilot light maintenance entirely and improves energy efficiency ^[3].