Pilot Won't Stay Lit After Replacing Thermocouple
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The Short Answer: When a new thermocouple does not fix a pilot that goes out, the original diagnosis was probably incomplete. Common reasons are a part that does not match the control (thermocouple versus thermopile), a sensor tip not sitting in the pilot flame, a weak or dirty pilot, drafts, or a gas control whose safety magnet no longer holds. Buying another sensor rarely helps; a measured diagnosis does.
Thermocouple or thermopile?
These look similar but are not interchangeable.
| Part | What it does | Typical output |
|---|---|---|
| Thermocouple: a single thin copper lead with a threaded nut | Holds the pilot safety valve open on older standing-pilot controls | Roughly 20 to 30 millivolts in a good flame |
| Thermopile: a thicker sensor, often with two leads or a plug | Powers electronic gas controls (usually with a status light) and their main valve | Several hundred millivolts |
A thermocouple installed on a control that needs a thermopile will not produce enough power. On sealed FVIR tanks, the sensor is often part of a manufacturer-specific pilot assembly, and a generic thermocouple may not fit the sealed inner door properly. Check the old part and the heater’s model number against the manufacturer’s parts list.
Other reasons the pilot still goes out
- Tip not in the flame. The pilot should wrap the top of the sensor. A bracket bent during installation can leave the tip beside or below the flame.
- Connection at the gas control. A thermocouple nut that is loose, cross-threaded, or overtightened can lose the tiny signal.
- Weak or dirty pilot. A small, yellow, or lifting pilot flame cannot heat even a new sensor properly. The pilot orifice may need cleaning.
- Draft. A missing inner door, a closet with no air, or downdrafts can blow the flame off the tip; see pilot light goes out in cold weather.
- Gas control. If the sensor produces normal output and the pilot still goes out when the knob is released, the safety magnet in the control has usually failed, and the control is replaced.
What you can check safely
- Follow the lighting label exactly, including how long to hold the knob after the pilot lights.
- Look through the sight glass: is the flame blue, steady, and touching the top of the sensor?
- Check that the outer and inner covers are in place and nothing is blocking the air intake.
- Note whether it goes out immediately on release, after a few minutes, or only when the main burner fires; pilot goes out when you turn the temperature dial covers the last case.
What the technician will check
A technician measures the sensor’s output with the pilot lit and compares it with what the control needs, checks the connection at the control, cleans or replaces the pilot, and checks draft and combustion air. If the readings are good and the control still will not hold, the gas control is replaced, which is licensed gas work in most areas. On FVIR tanks they often replace the complete pilot assembly with the manufacturer’s part; see replace a water heater pilot assembly.
Frequently Asked Questions
Q: Can a new thermocouple be defective? A: It happens but is uncommon. Installation problems such as poor placement in the flame or a bad connection are more likely. A technician can measure the output to know for sure.
Q: Can I jumper the thermocouple to test the gas valve? A: No. The thermocouple is a flame-safety device that shuts off gas if the pilot goes out. Bypassing it, even briefly, can release unburned gas.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
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