What a thermocouple does
A thermocouple is a thin metal probe with its tip sitting in the burner flame. When it gets hot, it generates a tiny electrical current, measured in millivolts. That current holds open an electromagnet inside the gas safety valve. If the flame goes out, the thermocouple cools, the current stops, and the valve snaps shut so gas doesn't keep flowing unburned.
That's why many burners have you push in and hold the knob for 20 to 60 seconds after lighting. You're manually holding the valve open until the thermocouple heats up enough to take over.
On grills, you'll usually find flame safety on rear rotisserie burners, sear burners, side burners and some full grills, especially commercial and some premium built-ins. They're also standard on gas fire pits, fireplaces and patio heaters, which fail the same way.
Why burners won't stay lit
In order of how often we see them.
Not holding the knob long enough
The thermocouple needs time to heat up. Letting go too early is the most common cause, especially on a cold burner.
Dirty thermocouple tip
Carbon and grease coat the tip and insulate it from the flame. It doesn't get hot enough to generate current.
Thermocouple out of position
If the tip gets bent out of the flame during cleaning, it won't heat properly.
Weak flame
A partially blocked burner or low gas pressure produces a small flame that doesn't fully cover the tip.
Learn moreWorn-out thermocouple
They wear out from years of heat cycling. Output drops below what the valve needs.
Loose connection or failed safety valve
The thermocouple threads into the valve. A loose or corroded connection breaks the circuit. Less often, the valve's magnet fails.
Learn moreWhat you can safely check
- Hold the knob in longer. Try a full 60 seconds after the burner lights before slowly releasing.
- Watch the flame while lit. It should wrap around the thermocouple tip, not just touch the side.
- With the burner off and cool, look at the tip. If it's coated in black carbon, a gentle wipe with fine steel wool or a scuff pad can help.
- Check that the tip is pointing into where the flame will be, not bent away.
- If the flame itself looks small or yellow, the burner probably needs cleaning before the thermocouple is blamed.
How we repair it
- 1
Check the flame first
A clean, full flame is the starting point. We clear the burner and confirm gas pressure before testing the thermocouple.
- 2
Measure output
We check the thermocouple's millivolt output with a meter while it's in the flame. That tells us whether it or the valve has failed.
- 3
Replace the thermocouple
A new thermocouple of the right length and fitting is installed, positioned in the flame, and threaded into the valve snug but not overtightened.
- 4
Replace the valve if needed
If the thermocouple tests good and the valve still won't hold, the safety valve is replaced and leak tested.
- 5
Test the shutoff
We light the burner, confirm it stays lit, then turn it off and confirm the valve closes in the expected time.
Thermocouple questions
How long should I hold the knob in?
It depends on the appliance. Most grill burners with flame safety need 20 to 60 seconds. Check your manual. On a cold day, it can take longer.
Can I replace a thermocouple myself?
On some appliances it's a simple threaded part. But it connects directly to the gas valve, and getting the fit and position wrong causes repeat failures or leaks. We recommend having a technician do it on grills and built-ins.
My burner goes out in the wind. Is that the thermocouple?
Wind can push the flame off the thermocouple and trip the safety. If it only happens on windy days, that's the system working. If it happens in calm weather, the thermocouple needs attention.
Is a thermocouple the same as a flame sensor?
They do similar jobs. Thermocouples generate their own small current from heat. Some appliances use flame rectification sensors or thermopiles instead, which work differently but fail in similar ways.
How much does thermocouple replacement cost?
It depends on the appliance and whether the valve also needs replacing. Call or text with the brand and model and we'll quote it.
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Burner won't stay lit?
Call or text us with the brand and model, whether it's a grill, fire pit, fireplace or heater. We'll get it holding a flame again.