The pilot light on a gas furnace is a small but vital flame that ignites the main burner. When the furnace pilot light keeps going out, the home can lose heat, the furnace may fail to start, and safety mechanisms can be triggered. This article explains common causes, diagnostic steps, repairs, and preventive maintenance in clear, actionable detail.
| Symptom | Possible Causes | Typical Fix |
|---|---|---|
| Pilot light goes out frequently | Drafts, dirty pilot orifice, weak thermocouple | Seal drafts, clean or replace orifice, replace thermocouple |
| Pilot lights but then dies when burner tries to ignite | Faulty gas valve, weak pilot flame, gas pressure issue | Inspect gas valve/pressure, adjust or replace components |
| Pilot lights but furnace won’t stay on | Thermocouple or flame sensor failure, wiring issue | Replace thermocouple/sensor, check wiring |
| Strong gas smell when pilot out | Gas leaking from valve or line | Shut off gas, ventilate, call licensed technician |
How A Furnace Pilot Light Works
The pilot light is a small, continuously burning flame that ignites the furnace’s main burners when the thermostat calls for heat. In older furnaces this flame is constant; many modern furnaces use an electronic ignition instead. The pilot flame interacts with a thermocouple or flame sensor to confirm the presence of flame and allow the gas valve to remain open.
Thermocouple is a heat-sensitive component that generates a tiny voltage when heated by the pilot flame; this voltage keeps the gas valve open. If the thermocouple cools, it signals the gas valve to close for safety.
Common Causes Why The Pilot Light Keeps Going Out
Drafts And Airflow Problems
Drafts from open windows, attic fans, or unsealed doors can blow a pilot flame out. Airflow changes around the furnace, such as from a nearby vent or exhaust, may also destabilize the flame. In many homes, simple air movement is the most common culprit.
Dirty Pilot Orifice Or Burner
Debris, dust, spider webs, or soot can partially block the pilot orifice or burner ports. A restricted or contaminated orifice weakens the pilot flame, making it easy to extinguish.
Faulty Thermocouple Or Flame Sensor
A worn or failing thermocouple may not produce enough millivoltage to keep the gas valve open. Flame sensors in electronic ignition systems can also become dirty or fail, causing the system to shut off gas as a safety precaution.
Weak Gas Pressure Or Faulty Gas Valve
Low or inconsistent gas pressure can produce a weak pilot flame. A partially closed or failing gas valve can limit gas flow to the pilot, creating unreliable ignition. Both conditions require professional measurement and adjustment.
Pilot Assembly Malalignment Or Damage
If the pilot assembly is misaligned, the flame may not properly contact the thermocouple or sensor. Corrosion, mechanical damage, or loose mounting can shift the flame away from sensitive components, causing shutdowns.
Condensation And Moisture
High humidity, condensation, or water infiltration can cause corrosion on the pilot assembly, thermocouple, and burner parts. Moisture can also temporarily extinguish the flame or interfere with sensing components.
Obsolete Or Converting Systems
Older furnaces reaching the end of their service life may have degraded parts and unreliable pilot systems. Furnaces converted improperly between natural gas and LP can have incorrect orifices and pressures leading to pilot issues.
How To Safely Diagnose The Problem
Safety is paramount. If a homeowner smells strong gas, do not attempt troubleshooting; immediately ventilate the area, avoid open flames or electronics that spark, shut off the gas at the main valve if trained to do so, and call the gas company or a licensed technician.
Visual Inspection
Check the pilot flame color and shape. A healthy pilot flame is steady and blue with a small inner cone touching the thermocouple. Yellow, flickering, or weak flames indicate contamination, improper pressure, or a dirty orifice.
Check For Drafts
Look for nearby open windows, doors, fans, or vents. Temporarily closing doors and windows around the furnace can confirm if drafts are extinguishing the pilot. Seal gaps in the furnace room to prevent air movement through the pilot area.
Listen And Smell
Listen for unusual hissing near the gas valve or smell for gas. Hissing can indicate a leak or valve issue. Any sign of a leak requires immediate professional attention and safe evacuation if necessary.
Test Thermocouple With Simple Actions
A loose thermocouple connection can be retightened by a trained homeowner, but replacing a failing thermocouple is often the actual fix. If the pilot stays lit when manually held with a tool or when the pilot assembly is repositioned, the thermocouple may be failing.
DIY Fixes Homeowners Can Try
Clean The Pilot Orifice And Burner
Turn off the furnace and gas supply, allow components to cool, and use compressed air or a fine brush to remove dust and webs from the pilot orifice. Avoid poking metal objects directly into the orifice to prevent damage.
Seal Drafts Around The Furnace Area
Seal gaps around doors, windows, and service panels. Install weather stripping on doors and use caulk for small gaps. Ensure exhaust fans and vents are balanced to avoid creating negative pressure near the furnace.
Replace A Thermocouple
Thermocouples are inexpensive and commonly replaced. After shutting off gas and power, unscrew the thermocouple at the gas valve and remove the mounting bracket. Install the new thermocouple ensuring the tip sits directly in the pilot flame and tighten connections.
Light The Pilot Properly
Follow the furnace’s lighting instructions. Typically this involves setting the gas valve to “pilot,” pressing and holding the valve knob to allow gas flow, lighting the pilot, and holding the knob until the thermocouple heats (usually 30-60 seconds). Release slowly to confirm the pilot stays lit.
When To Call A Professional
Call a licensed HVAC or gas technician if the pilot light continues to go out after basic cleaning and thermocouple replacement, if there is a gas odor, if the gas valve or pressure appears faulty, or if the furnace is older and showing multiple failure signs.
Professional diagnostics include gas pressure measurement, valve testing, electrical checks, and inspecting for hidden leaks or conversion errors. Technicians can also advise on replacing the furnace with a modern electronic ignition system for improved safety and efficiency.
Preventive Maintenance To Keep The Pilot Lit
- Schedule annual professional furnace inspections and cleanings to maintain pilot and burner condition.
- Keep the furnace area clean and free of dust, debris, and combustible materials.
- Verify proper ventilation and avoid creating strong draft paths near the furnace.
- Replace worn components such as thermocouples, pilot tubes, and gas valves proactively when signs of wear appear.
Consider Upgrading From A Standing Pilot
Homes with older furnaces that rely on a continuous pilot light use gas continuously and are less efficient than modern systems. Converting to an electronic ignition or replacing the furnace with a modern unit eliminates pilot-related problems and reduces gas usage.
Electronic ignitions, such as intermittent pilot or hot surface ignitors, light the main burners only when needed. This change increases efficiency and reduces maintenance related to pilot lights.
Cost Estimates And Practical Advice
Basic maintenance like cleaning the pilot orifice may be low-cost or free for a handy homeowner. Thermocouple replacement typically ranges from $20 to $80 for the part and $75 to $200 with professional labor. Gas valve replacements and pressure repairs can cost several hundred dollars.
Replacing an older furnace with a new high-efficiency model is a larger investment but often pays back through energy savings and reduced repair bills. A professional can provide cost-benefit analysis and recommend options suited to the home.
Safety Reminders When Dealing With Pilot Issues
- If There Is A Strong Gas Odor: Evacuate, ventilate, avoid sparks, and call the gas company or emergency services.
- Do Not Attempt Complex Gas Valve Repairs: Only licensed technicians should work on gas valves, lines, and pressure regulators.
- Regular Maintenance: Annual inspections reduce the risk of dangerous failures and ensure safe operation.
Quick Troubleshooting Checklist
- Check for drafts or open windows near the furnace.
- Inspect pilot flame color—blue is healthy; yellow or flickering indicates a problem.
- Clean the pilot orifice and burner area gently with compressed air.
- Tighten or replace a loose or worn thermocouple.
- If problems persist or a gas smell is present, contact a licensed technician immediately.
Resources And Further Reading
For detailed safety guidance and manufacturer instructions, consult furnace owner manuals and reputable sources such as the U.S. Consumer Product Safety Commission, local utility safety pages, and HVAC trade associations. Licensed professionals and local utility emergency lines are primary contacts for hazardous situations.
Addressing a pilot light that keeps going out often restores reliable heating and prevents unsafe gas exposure. Basic cleaning and component replacement fix many cases, while professional help is essential for gas leaks, valve issues, or complex repairs.