Furnace Pilot Light Problems: Causes and Repair Cost

Standing Pilot Light Problems

Furnaces manufactured before the early 1990s typically use a standing pilot light, a small gas flame that burns continuously to ignite the main burner when the thermostat calls for heat. While simple and reliable, standing pilots are subject to several common problems that prevent them from staying lit.

Thermocouple failure is the most common cause of a pilot light that will not stay lit. The thermocouple is a small metal probe positioned in the pilot flame that generates a tiny electrical voltage when heated. This voltage holds the gas valve open, allowing gas to flow to the pilot. When the thermocouple wears out, it can no longer generate enough voltage to keep the gas valve open, and the pilot goes out as soon as you release the pilot button. Thermocouple replacement costs $100 to $200 including parts and labor. The part itself costs $10 to $25, with the rest being the service call fee.

Dirty pilot orifice reduces the flame size to the point where it cannot adequately heat the thermocouple. Dust, lint, and rust particles can partially block the tiny opening where gas exits to feed the pilot flame. Cleaning the pilot orifice costs $80 to $150 as part of a service call and restores a strong, stable pilot flame.

Drafts can blow out the pilot light, particularly in basements with windows or in utility closets near exterior doors. If your pilot light goes out periodically but relights easily each time, air movement near the furnace may be the cause. Sealing drafts near the furnace or installing a wind shield around the pilot assembly can resolve the issue without any parts replacement.

Gas supply interruption will extinguish the pilot and prevent relighting until gas service is restored. If the pilot will not light at all and you do not smell gas, check the gas valve at the furnace (it should be in the "Pilot" or "On" position) and verify that other gas appliances in the home are working. If no gas appliances work, contact your gas utility.

Modern Flame Sensor Problems

Furnaces manufactured since the early 1990s use electronic ignition (hot surface ignitor or spark ignition) instead of a standing pilot. These systems include a flame sensor, a thin metal rod positioned in the main burner flame, that confirms the burner has successfully ignited. If the flame sensor does not detect a flame within a few seconds of the gas valve opening, the control board shuts down the system as a safety precaution.

Dirty flame sensor is the most common problem with modern ignition systems. Over time, the metal rod develops a coating of oxidation that insulates it from the flame, reducing the electrical signal it sends to the control board. When the signal drops below the threshold, the control board interprets it as "no flame detected" and shuts the gas valve, even though the burner is actually lit. You see the burner light, burn for 3 to 5 seconds, and then shut off. This cycle repeats several times before the furnace locks out.

Cleaning the flame sensor is a simple fix that costs $80 to $200 for a professional service call. The technician removes the sensor (usually held by a single screw), cleans the metal rod with fine emery cloth or steel wool to remove the oxidation layer, reinstalls it, and tests the system. A clean flame sensor restores reliable operation immediately.

Failed flame sensor that does not respond even after cleaning needs replacement. The sensor itself costs $15 to $40, and the total repair with labor runs $150 to $300. Flame sensors typically last 5 to 10 years before cleaning is no longer sufficient and replacement becomes necessary.

Cracked or failed ignitor prevents the burner from igniting in the first place. If you hear the furnace startup sequence but the burner never lights, the ignitor may have failed. This is a separate issue from flame sensor problems but produces similar no-heat symptoms. See our ignitor replacement cost guide for details.

How to Relight a Standing Pilot Light

If your furnace has a standing pilot light that has gone out, you can attempt to relight it yourself before calling for service. Follow these steps carefully.

Turn the gas control knob to the "Off" position and wait at least 5 minutes for any accumulated gas to dissipate. If you smell gas strongly, do not attempt to relight the pilot. Instead, leave the area, open windows, and call your gas utility from outside the home.

After waiting, turn the gas control knob to the "Pilot" position. Press and hold the knob down (or press a separate pilot button if your model has one) while holding a long lighter or match to the pilot opening. The pilot should light within a few seconds. Continue holding the knob down for 30 to 60 seconds after the pilot lights to allow the thermocouple to heat up and generate enough voltage to hold the gas valve open independently.

Release the knob slowly. If the pilot stays lit, turn the gas control knob to the "On" position and set the thermostat to call for heat. If the pilot goes out when you release the knob, wait another minute and try again, holding the knob down longer. If it goes out after three attempts, the thermocouple is likely failed and needs professional replacement.

When to Call a Professional vs DIY

Some pilot light and flame sensor issues are safe for homeowners to troubleshoot, while others require professional service.

Safe for DIY: Relighting a standing pilot light following the steps above is a routine task that any homeowner can perform safely. Replacing the air filter is also safe and should be done regularly regardless of furnace issues. Checking that the gas valve is in the correct position and that the thermostat is set properly are basic steps to try before calling for service.

Requires a professional: Any work involving gas line connections, electrical components, or opening the furnace cabinet beyond basic filter access should be handled by a licensed HVAC technician. Cleaning a flame sensor requires removing a component and handling the furnace internals, which most manufacturers recommend leaving to a professional. If you smell gas that does not dissipate after a few minutes with the gas valve in the "Off" position, leave the home immediately and call your gas utility's emergency line from outside.

Repeated problems warrant investigation: If your pilot light goes out more than twice per heating season or your flame sensor needs cleaning more than once per year, the furnace likely has an underlying issue that a service call should diagnose. Frequent flame sensor fouling can indicate burner alignment problems, gas quality issues, or a cracking heat exchanger that is introducing contaminants into the combustion chamber.

Electronic Ignition Types Explained

Modern furnaces use one of two electronic ignition methods instead of a standing pilot. Understanding which type your furnace has helps you communicate with a technician and understand repair quotes.

Hot surface ignition (HSI) is the most common type in residential furnaces manufactured since the late 1990s. The ignitor is a small element made of silicon carbide or silicon nitride that heats to 1,800 to 2,500 degrees Fahrenheit when the control board energizes it. Once the ignitor reaches temperature (which takes 15 to 60 seconds), the gas valve opens and the gas ignites on contact with the glowing element. Silicon nitride ignitors are more durable and have largely replaced silicon carbide in newer furnaces. Replacement cost runs $150 to $350 including parts and labor.

Intermittent pilot ignition uses a spark to light a small pilot flame, which then ignites the main burner. Unlike a standing pilot, this pilot only lights when the thermostat calls for heat and extinguishes after the heating cycle ends. The system uses less gas than a standing pilot because the pilot is not burning continuously. This type is less common in newer furnaces but is still found in some mid-tier models. The spark module costs $150 to $300 to replace if it fails.

Should You Upgrade from a Standing Pilot

If your furnace still uses a standing pilot light, it is at least 30 years old and well beyond the typical lifespan for any furnace type. While the pilot system itself may continue to function with thermocouple replacements, the furnace as a whole is operating at significantly reduced efficiency (typically 60% to 70% AFUE compared to 90%+ for modern units) and carries elevated safety risks due to its age.

Replacing a pilot-light-era furnace with a modern electronic ignition model eliminates the gas waste of the continuous pilot flame (which costs $5 to $10 per month), improves efficiency by 25% to 35%, reduces carbon monoxide risk, and provides reliable, maintenance-free ignition for years. The investment in a new furnace at this stage is less about the pilot light specifically and more about the overall age, safety, and efficiency of the entire system.