Pilot Light Won't Stay Lit? How to Test and Replace a Thermocouple

Why does the pilot light go out when I release the knob?

Because the thermocouple is no longer telling the gas valve that the flame is lit.

Hold the control knob in and you are manually forcing the gas valve open. Release it, and the valve will only stay open if it is receiving a small electrical signal proving the pilot flame is burning. That signal comes from the thermocouple. A weak or dead thermocouple cannot produce it, so the valve shuts and the pilot dies — exactly as it is designed to.

This is a safety feature working correctly. It is stopping unburnt gas from filling your kitchen. The fault is the sensor, not the safety system.

What does a thermocouple actually do?

A thermocouple is a temperature sensor made of two dissimilar metals joined at a tip. When that tip sits in the pilot flame, the heat generates a tiny electrical current — typically 25 to 30 millivolts. That current energises an electromagnet inside the gas valve, holding it open.

No flame means no heat, which means no voltage, which means the valve closes within seconds. It is a purely mechanical safety chain requiring no mains power, which is why it has been used on commercial gas equipment for decades.

How do I know it's the thermocouple and not something else?

Work through these in order. The pattern of the fault tells you a lot.

• Pilot lights, but dies the instant you release the knob — classic failed thermocouple.
• Pilot lights, burns for a minute or two, then dies — usually a weak thermocouple that can no longer hold the valve, or a dirty tip.
• Pilot flame is weak, yellow or lifting away from the tip — the problem is the pilot burner or a blocked orifice, not the thermocouple. The thermocouple tip must sit properly in the flame to work.
• Pilot won't light at all — gas supply, blocked pilot orifice, or ignition fault. Not the thermocouple.
• Pilot holds fine, but the main burner won't fire — that points to the gas control valve or, on some appliances, a thermopile rather than a thermocouple.

How do I test a thermocouple?

With a multimeter set to millivolts DC:

1. Disconnect the thermocouple lead from the gas valve.
2. Light the pilot and let the tip heat for at least 30 seconds — it needs to reach full temperature.
3. Measure across the thermocouple tip and its connector.

A healthy thermocouple reads roughly 25–30 mV. Below about 15 mV it will not reliably hold the valve open and should be replaced. A reading of zero means it has failed completely.

No multimeter? The behaviour test is nearly as reliable: if the pilot dies within a few seconds of releasing the knob, every time, and the flame itself looks strong and blue, replace the thermocouple.

Can I clean it instead of replacing it?

Sometimes — it is worth five minutes before you spend anything.

Soot and carbon build up on the tip and insulate it from the flame, so it never gets hot enough to generate full voltage. Gently clean the tip with fine steel wool or a soft abrasive cloth, and check the connection into the gas valve is tight — a loose connection loses signal.

Also check the tip is actually sitting in the flame. Thermocouples get knocked out of position during cleaning, and one that is sitting beside the flame instead of in it will fail even though the part is fine.

If cleaning and repositioning don't fix it, the thermocouple is worn out. They are consumable parts — constant heat cycling degrades them, and they are not repairable.

Which thermocouple do I need?

Three things decide it:

Most commercial gas equipment takes a universal thermocouple kit, which comes with adaptors covering the majority of valve fittings. Our SIT Super Universal kits are stocked in 600mm, 900mm and 1500mm at £18.99, including interrupted, nickel-tip and aluminised-tip versions. There is also a 600mm M9x1 extension if your run is slightly short.

Browse all thermocouples — 63 parts in stock, compatible with Falcon, Lincat, Blue Seal and most major commercial brands.

How do I fit a replacement thermocouple?

1. Isolate the gas at the appliance and let everything cool.
2. Unscrew the thermocouple nut from the gas valve — usually a small spanner, finger-tight plus a quarter turn on reassembly.
3. Release the tip from its bracket at the pilot assembly.
4. Route the new thermocouple along the same path. Avoid sharp bends and keep it away from hot surfaces.
5. Seat the tip so it sits directly in the pilot flame, in the same position as the old one. This is the step most people get wrong.
6. Hand-tighten the nut at the valve, then a quarter turn with a spanner. Do not overtighten — crushing the connection kills the signal.
7. Restore gas, light the pilot, hold for 30 seconds, release. It should stay lit.

Gas work must be carried out by a competent person. In the UK, work on commercial catering gas appliances must be done by a Gas Safe registered engineer qualified for commercial catering.

What if a new thermocouple doesn't fix it?

If a known-good thermocouple still won't hold the pilot, the fault is downstream:

• The electromagnet in the gas valve has failed. It can no longer hold on 25–30 mV. The valve needs replacing — see gas control valves.
• The pilot flame is too weak to heat the tip properly. Clean or replace the pilot burner and check the orifice is clear.
• Your appliance uses a thermopile, not a thermocouple. They look similar but are not interchangeable — a thermopile puts out 250–750 mV. If yours has two wires into a control board, it is a thermopile.

Frequently asked questions

How long does a thermocouple last?

There is no fixed lifespan — it depends on how hard the appliance is worked. In a busy commercial kitchen, thermocouples are consumable items and failure is routine, not a sign of a bad appliance. Many kitchens keep a spare on the shelf precisely because the failure always happens at service.

Can I use a thermopile instead of a thermocouple?

No. They work on the same principle but produce very different voltages — around 25–30 mV versus 250–750 mV — and are not interchangeable. Fitting the wrong one will not work.

Is a universal thermocouple as good as the manufacturer's part?

For most commercial equipment, yes. A universal kit with the correct length and thread does exactly the same job. What matters is length, thread and that the tip sits correctly in the flame.

How quickly can I get one?

We hold thermocouples in UK stock. Order before 10am for next-day delivery, and delivery is free on orders over £25. If you are unsure which one fits, call 073 8051 5838 with your appliance make and model and we will identify it for you.

Still not sure which part you need?

Send us your appliance make, model and a photo of the old part. Our team fits and troubleshoots this equipment every day and will tell you exactly what you need.

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