Pilot Light Will Not Stay Lit

Gas safety first

Working around gas appliances requires careful attention to safety. Raw fuel gas inside your living space is dangerous. Safety devices exist to stop the flow of gas whenever a steady flame is missing.

Before you look at the pilot burner, turn the gas control knob to the off position. Shut off the manual gas valve on the supply pipe as well. If at any point you smell gas, stop what you are doing immediately. Do not flip light switches, do not unplug appliances, and do not use a phone inside the house. Leave the building at once and call your gas company from outside.

What the thermocouple does

A thermocouple is a simple heat sensor placed right next to the pilot flame. Its job is to confirm that the pilot is burning before the main gas valve is allowed to open.

When the pilot flame heats the tip of the thermocouple, the heat produces a tiny electrical voltage measured in millivolts. This small voltage travels through the metal probe down to the gas valve. The voltage powers an internal magnet that holds the pilot valve open. If there is no heat, there is no voltage. Without voltage, the gas valve closes automatically so unburned gas cannot leak into the room.

The tell-tale sign of a bad sensor

There is a classic sign that points directly to a failing sensor. When you light the pilot, you must manually push down and hold the control button. This manual pressure physically forces the internal gas port open so gas reaches the burner tip.

Once you light the pilot, you hold the button down so the tip can warm up. When you release the button, the electrical voltage from the heated sensor is supposed to take over and hold the valve open. If the pilot lights while you hold the button, then goes out when you let go, the sensor is likely weak, dirty, or broken. The valve does not detect enough voltage, so it snaps shut.

Thermocouple or thermopile

Not every pilot assembly uses the exact same sensor. Some systems use a standard thermocouple, while others use a larger device called a thermopile.

A thermocouple produces a very low voltage, typically around 30mV. It is designed solely to keep the pilot safety magnet energized inside the gas valve. For example, a White Rodgers H06E-18 thermocouple produces 30mV and is rated for temperatures up to 1450°F Max.

A thermopile, sometimes called a pilot generator, is made of several thermocouples joined together. Because of this, it produces much more electrical energy. A thermopile often has an output of 600 mV to 750 mV, with a rated voltage of 750mV. That higher voltage is strong enough to power both the pilot safety magnet and an entire millivolt control circuit, such as an off-grid wall thermostat. For example, a Resideo Q313A1170 thermopile has a Cold Junction rating of 780 F, resistance of 2.89 ohms, and is used with Q382 Pilot Burners.

What to match on a new one

If you need a replacement sensor, you must match the physical and electrical design of the part on your equipment.

First, confirm whether your unit uses a standard millivolt thermocouple or a higher output thermopile. You cannot swap one type for the other because their electrical outputs are completely different.

Second, check the length of the copper tube or cable. The replacement must be the same length or a little longer so it can reach from the pilot bracket to the control valve without stretching or kinking. For example, you can find parts measuring 18" or 35" in length.

Third, inspect the connection style. A thermocouple usually attaches to the valve with a threaded nut, such as an 11/32" 32 Male Nut. A thermopile often connects using spade terminals on the end of the wiring, and it may include a 1/2" thermopile attaching nut to secure the probe into the pilot burner bracket.

Steps for inspecting the pilot area

Before replacing parts, take a close look at the physical burner setup to see if something simple is preventing normal operation.

1. Turn off the gas supply and allow all metal components around the burner to cool completely.
2. Check the position of the sensor tip to ensure the pilot flame fully engulfs the top third of the probe.
3. Look for heavy soot or corrosion buildup on the metal probe that might prevent the flame heat from transferring.
4. Check the fitting connection at the gas valve to make sure the nut or terminal screws are clean and snug.
5. If the pilot still drops out after cleaning and tightening, install a matching replacement sensor.