Pilot Light Won’t Stay Lit After Replacing Thermocouple: Troubleshooting Guide

When a furnace pilot light won’t stay lit after replacing the thermocouple, diagnosing the issue quickly restores heat and safety. This guide explains how the pilot system works, common causes after a thermocouple replacement, step-by-step troubleshooting, safety checks, cost expectations, and when to call a technician. It focuses on practical, accurate actions a homeowner or DIYer can take while emphasizing safety.

Symptom	Likely Causes	Quick Fix
Pilot lights but goes out	Incorrect thermocouple type/position, dirty pilot orifice, loose connection	Check alignment, tighten connection, clean orifice
Ignites then dies immediately	Faulty gas valve or weak thermocouple, pilot flame not enveloping tip	Replace thermocouple or test valve; adjust flame
No pilot flame	Gas supply issue, clogged orifice, regulator problem	Verify gas, clean or replace orifice, call pro if needed

How The Pilot Light And Thermocouple Work

The pilot light provides a small continuous flame to ignite the main burner when heat is needed. The thermocouple is a safety sensor that generates a millivolt electrical signal when heated by the pilot flame. That signal keeps the gas valve open. If the thermocouple cools or fails, the valve closes and the pilot goes out.

Understanding these components helps identify whether the problem is mechanical, electrical, or related to gas flow and flame geometry.

Common Causes When Pilot Won’t Stay Lit After Replacing Thermocouple

Incorrect Thermocouple Type Or Length

Replacement thermocouples must match the furnace specifications. Using the wrong type, length, or orientation can prevent proper heating of the sensor tip, causing the pilot to drop out.

Poor Thermocouple Positioning

If the thermocouple tip isn’t fully enveloped by the pilot flame or is positioned too far from the flame, it will not generate sufficient millivolts to hold the gas valve open.

Loose Or Corroded Connections

A loose nut at the gas valve or corroded wiring (on millivolt systems with connecting wires) reduces the electrical path and prevents the valve from sensing the thermocouple signal.

Dirty Or Clogged Pilot Orifice

Soot, dust, or insect nests in the pilot orifice cause a weak or erratic flame that doesn’t heat the thermocouple properly. Cleaning often restores stable operation.

Weak Or Defective Thermocouple

New thermocouples can be faulty. Manufacturing defects or damage during installation may cause insufficient voltage output even if the part appears intact.

Faulty Gas Valve Or Pilot Valve

If the gas valve is sticking or the internal pilot valve is failing, the valve may close despite a healthy thermocouple signal. This requires component replacement or professional repair.

Insufficient Gas Pressure Or Regulator Issues

Low incoming gas pressure or a failing regulator results in a small, unstable pilot flame. Even with correct thermocouple operation, the flame may be too weak to heat the sensor.

Tools And Safety Items Needed For Troubleshooting

• Adjustable wrench and small open-end wrenches
• Needle-nose pliers and a thin wire or compressed air for cleaning
• Multimeter (millivolt measurement) for thermocouple output
• Manufacturer’s replacement thermocouple or OEM part
• Work gloves and safety glasses
• Soap-and-water solution for gas-leak check

Step-By-Step Troubleshooting

1. Confirm Basic Safety And Gas Supply

Before any work, turn off power and follow furnace shutdown procedures. Verify gas is on and other gas appliances work. If the smell of gas is present, leave the area and call the gas company or emergency services.

2. Inspect Thermocouple Installation

Check that the thermocouple is the correct type and length for the furnace and that the tip is centered in the pilot flame. The tip should be fully enveloped by a stable blue flame. Tighten the nut at the gas valve snugly but avoid over-tightening.

3. Clean The Pilot Orifice And Assembly

Turn off the gas and use a thin wire, compressed air, or approved cleaner to remove debris from the pilot orifice. Reassemble and relight the pilot following manufacturer instructions. A clean orifice often restores flame stability.

4. Check The Flame Shape And Color

A healthy pilot flame is mostly blue, centered on the thermocouple tip, and steady. A yellow, flickering, or weak flame suggests contamination, airflow changes, or gas pressure problems.

5. Test Thermocouple Output With A Multimeter

Set the multimeter to millivolts. Light the pilot and hold the thermocouple in the flame. A typical functioning thermocouple reads between 20–30 millivolts. Values significantly lower indicate a bad thermocouple or poor flame contact.

6. Check For Loose Or Corroded Connections

Inspect and clean the thermocouple connection at the gas valve. Remove corrosion and ensure the connection is tight. On systems with millivolt wiring, check wire continuity and terminal connections.

7. Evaluate The Gas Valve

If the thermocouple produces adequate millivolts and the pilot still extinguishes, the gas valve may be failing. Testing the valve requires specific procedures and sometimes a technician’s tools; replacement is common when valves stick or leak.

8. Verify Gas Pressure And Regulator Function

If other steps fail, have a qualified technician measure inlet pressure and regulator performance. Low pressure requires utility or regulator adjustments and is not a DIY fix in most cases.

When To Replace Parts

Replace the thermocouple if measurements are below the expected millivolt range or if the part is physically damaged. Replace the pilot orifice if it is corroded or deformed. Replace the gas valve only after confirming the thermocouple and flame are correct, or if the valve shows internal failure symptoms.

Part	Typical DIY Cost Range	Notes
Thermocouple	$10–$60	Costs vary by length and OEM specification
Pilot Orifice	$5–$25	Often included with pilot assembly kits
Gas Valve	$150–$500+	Labor increases total; professional replacement recommended

Safety Precautions

Always shut off gas and power before disassembling pilot components. Use soapy water to check for gas leaks after reassembly. If gas smells persist or if there is inability to stop a leak, evacuate the premises and contact emergency services or the gas company.

Do not attempt repairs beyond user-level tasks (like replacing a gas valve or adjusting regulator pressure) unless trained and qualified. Improper work risks fire, explosion, carbon monoxide, and injury.

When To Call A Professional

Contact a licensed HVAC technician if the pilot remains unstable after cleaning, checking thermocouple output, and tightening connections. Also call a professional for gas pressure issues, suspected gas leaks, or when replacement of a gas valve or regulator is required.

Professional intervention ensures compliance with codes, safe handling of gas components, and correct diagnosis of complex issues like intermittent valve faults or combustion problems.

Preventive Maintenance Tips

• Schedule annual furnace service to clean pilot assemblies and test safety sensors.
• Use OEM-spec parts when replacing the thermocouple and pilot components.
• Keep the furnace area free of dust and insects; install a screen if pests invade the pilot housing.
• Periodically check pilot flame and tighten connections if vibration loosens fittings.

Search Keyword Integration And Practical Takeaways

This guide addresses the core query “furnace pilot light won’t stay lit after replacing thermocouple” by explaining mechanical causes, testing steps, safety checks, and replacement guidance. Key actions include verifying thermocouple position, measuring millivolts, cleaning the pilot orifice, and checking valve operation.

Following these steps helps distinguish between a simple installation error and a component failure that requires professional service, improving safety and reducing downtime.

Additional Resources And References

For further reading, consult furnace manufacturer manuals for model-specific thermocouple specifications and installation diagrams. Trusted HVAC resources and local utility safety pages provide guidance on gas leak response and certified technician locators.