Furnace Pilot Light Troubleshooting

Your furnace’s pilot light has gone out, the house is getting cold, and you are not sure whether to relight it yourself or call someone. Standing pilot furnaces are mechanically simple, and most no-light or won’t-stay-lit problems trace back to one of five causes: no gas, a dirty orifice, a failed thermocouple, a faulty gas valve, or a blockage in the pilot tube. This guide walks you through safe diagnosis step by step, with realistic US repair costs so you know when a DIY fix makes sense and when to bring in a pro.

How a Standing Pilot Furnace Works

Before troubleshooting, it helps to understand what you are actually looking at.

A conventional standing pilot furnace uses a small, continuous flame—usually burning 500 to 1,200 BTU per hour—to ignite the main burner when the thermostat calls for heat. That tiny flame does two jobs:

1. Ignition: It lights the main gas burner, which on a typical residential furnace delivers 40,000 to 120,000 BTU/hour of heating input.
2. Safety signaling: It heats a thermocouple (or occasionally a thermopile), a pair of dissimilar metal wires that generate a small electrical current—roughly 20 to 30 millivolts for a thermocouple—which holds the gas valve open. If the flame dies, the current stops, and the valve closes within seconds to prevent gas from accumulating.

That safety circuit is the reason most pilot problems present as “the flame lights but won’t stay on.” The flame itself is fine; the sensor telling the valve to stay open is not.

All gas-fired central furnaces sold in the US are certified to ANSI Z21.47 and must be installed per NFPA 54 (National Fuel Gas Code) and local mechanical codes. Those standards are also why manufacturers place the gas control knob, pilot access, and lighting instructions behind a specific panel—never bypass them.

Note on modern equipment: Furnaces manufactured in recent decades use electronic ignition—either a hot surface igniter (HSI) or intermittent pilot (IP)—instead of a standing pilot. If your furnace has no visible pilot light and no gas knob labeled PILOT/OFF/ON, you have an electronic ignition system and this guide’s relighting steps do not apply. See our furnace troubleshooting section for ignition-specific diagnostics.

Safety First: Before You Touch Anything

• Smell gas? Stop. Do not flip switches, use a phone, or light anything. Leave the house and call your gas utility’s emergency line from outside.
• Shut off the gas valve at the furnace (the lever perpendicular to the pipe) before removing any panels.
• Let the furnace cool for at least 5–10 minutes before relighting so residual gas can dissipate.
• Have a working CO alarm on every level of the home. The UL 2034 standard governs CO alarms, and the NFPA recommends one outside each sleeping area. A yellow, lazy, or lifting flame is a carbon monoxide warning sign—not a tuning issue.
• Never use a lighter, match, or paper to reach into a burner compartment you cannot see clearly.

If at any point the smell of gas persists after the valve is off, treat it as an emergency.

Step-by-Step Furnace Pilot Light Troubleshooting

Work through these in order. Most homeowners find the problem within the first three steps.

Step 1: Confirm the Gas Supply Is Actually On

This sounds obvious and is the single most common “failure.”

• Check that the manual shutoff valve on the gas line to the furnace is parallel to the pipe (open), not perpendicular (closed).
• If your home has propane, check the tank level. A tank below 20% can deliver inconsistent pressure and cause the pilot to starve.
• If other gas appliances (water heater, range) also fail, the issue is upstream—call your utility.

Step 2: Check for a Dirty or Clogged Pilot Orifice

A pilot orifice is a pinhole roughly the diameter of a sewing needle. Dust, rust flakes, spider webs, and grease from a nearby laundry room can plug it.

• Look for a flame that is too small, yellow, or sputtering.
• Clean the orifice with compressed air or a single strand of copper wire. Do not enlarge the hole with a drill bit or paperclip—this alters the gas-to-air ratio and is a code violation.
• Check the pilot tube for kinks or cracks.

Step 3: Inspect the Thermocouple (The Usual Suspect)

If the pilot lights while you hold the knob down but dies the moment you release it, the thermocouple is the problem roughly 70% of the time.

• Position: The thermocouple tip should sit in the flame, about 1/8 to 1/4 inch from the pilot hood, with the top 3/8 to 1/2 inch engulfed in the flame.
• Condition: Look for a burned tip, soot buildup, or a melted lead. Clean lightly with fine steel wool (0000 grade)—never sandpaper, which removes the protective oxide layer.
• Continuity: A technician can test output with a multimeter; anything under 15 millivolts under load means replace it.

Cost: The thermocouple itself is $10–$30 at any hardware store. A technician will charge $150–$250 installed, including the diagnostic visit.

Step 4: Test the Thermopile and Gas Valve

Some furnaces use a thermopile instead of a thermocouple, generating 250–750 millivolts to power both the valve and the thermostat circuit.

• If the thermopile tests within spec but the burner still won’t fire, the gas valve may be stuck closed or the valve coil may have failed.
• A gas valve is not a homeowner-replaceable part. It requires gas line disconnect, leak testing with a manometer or soap solution, and code-compliant reconnection.

Cost: Gas valve replacement runs $300–$600 installed for most residential furnaces, plus the service call.

Step 5: Relight the Pilot Correctly

Once you have addressed the cause, relight using the manufacturer’s sequence:

1. Set the thermostat to OFF and the gas knob to OFF (or PILOT).
2. Wait 5 minutes.
3. Hold the knob down (or press the red reset button) and apply flame to the pilot hood.
4. Keep holding for 30–60 seconds after the flame is established—this gives the thermocouple time to heat and generate voltage.
5. Release slowly. The flame should remain lit. If it doesn’t, repeat once; if it still fails, stop and move to diagnosis.

Common Causes at a Glance

Pilot Light vs. Electronic Ignition: What You Should Know

Standing pilots are essentially extinct in new equipment in the US. Since the 1990s, manufacturers have shifted to electronic ignition for three reasons:

• Efficiency: A standing pilot burns gas 24/7/365, adding roughly $20–$60 per year to a gas bill. Electronic ignition eliminates that waste and helps furnaces reach 80% to 98.5% AFUE.
• Reliability: No standing flame means no thermocouple to fail.
• Federal rules: ENERGY STAR-certified furnaces must meet a minimum 90% AFUE (for gas, in most climate zones per current DOE standards), and no standing-pilot unit qualifies.

If your furnace is 20+ years old with a standing pilot, repeated repairs are a signal to start budgeting for replacement. A new 80% AFUE furnace typically costs $2,500–$4,500 installed, while a high-efficiency 96–98.5% AFUE condensing unit runs $4,500–$8,000 installed, before any available federal tax credits or utility rebates.

When to Call a Professional

Call a licensed HVAC technician—and skip the DIY steps—if any of the following apply:

• You smell gas at any point.
• The pilot lights but the main burner never fires.
• The flame is yellow, orange, or lifting off the burner.
• You see soot, rust, or white chalky residue around the burner or flue.
• Your CO alarm has triggered.
• The furnace has a cracked heat exchanger (a safety hazard, not a repair).
• The gas valve needs replacement.

A professional service call typically costs $75–$150 for the visit, with a full diagnostic running $100–$200. Many companies waive the diagnostic fee if you approve the repair. For a broader look at what repairs run in your region, see our furnace repair cost breakdowns.

Preventing Future Pilot Problems

• Schedule annual maintenance in early fall. A tune-up ($100–$200) includes cleaning the pilot assembly, checking gas pressure, and inspecting the heat exchanger.
• Keep the furnace room clean. Laundry lint and pet hair are pilot-orifice killers.
• Replace the filter every 1–3 months. Restricted airflow overheats the heat exchanger and can trip the high-limit switch.
• Seal return-air leaks near the furnace. Negative pressure in a basement pulls dust into the combustion air supply.
• Don’t store paint thinner, gasoline, or aerosol cans near the furnace—vapors are flammable and can cause flame rollout.

Frequently Asked Questions

Why won’t my furnace pilot light stay lit after I release the button? If the pilot goes out the moment you release the gas knob, the thermocouple is almost always the culprit. The thermocouple is a small safety sensor that generates 20–30 millivolts of electricity when heated by the pilot flame. If the tip is dirty, bent, or worn out, the gas valve never receives the signal to stay open. Clean the tip with fine steel wool and confirm it sits in the flame, or replace the thermocouple ($10–$30 for the part, $150–$250 installed).

Is it safe to relight my own furnace pilot light? Yes, relighting a standing pilot is a routine homeowner task if you follow the manufacturer’s instructions on the access panel and never force the gas knob. Stop immediately and call a licensed technician if you smell gas, if the pilot lights but the main burner does not, if the flame is yellow instead of blue, or if you find soot or corrosion around the burner assembly.

How much does it cost to replace a furnace pilot light assembly? A full pilot assembly replacement typically runs $150–$350 installed, including the orifice, thermocouple, tubing, and labor. A standalone thermocouple replacement is the cheapest fix at $150–$250 total, while a failed gas valve—the component the pilot protects—can cost $300–$600 installed. Diagnostic or service call fees usually add $75–$150 on top.

The Bottom Line

Most furnace pilot light failures come down to a dirty orifice or a tired thermocouple, and both are inexpensive to fix. Start with the gas supply, clean the pilot assembly, check thermocouple placement, and relight using the exact sequence printed on your furnace’s access panel. If the flame goes out on release, replace the thermocouple; if the pilot holds but the main burner never lights, stop and call a licensed technician—that symptom points to a gas valve or safety control issue that is neither safe nor legal for a homeowner to repair. And if your furnace still runs on a standing pilot, treat every repair as a countdown: modern 90%+ AFUE equipment will cut your gas use, eliminate the pilot entirely, and pay back the difference over its service life. For more diagnostic help before you call anyone, browse our full library of HVAC troubleshooting guides.