If the pilot lights but goes out shortly after you release the control knob, the thermocouple is the most likely cause. This small safety sensor has to heat up enough to prove a flame is present before it will let gas keep flowing to the pilot — if it is worn, misaligned, or dirty, it cools and cuts the gas before you have finished the lighting sequence, even though the pilot itself was burning fine.
How the safety system works
A standing pilot light is monitored by a thermocouple (or on newer units, an electronic ignition sensor) specifically so that gas cannot keep flowing if the pilot has gone out — without this, unburned gas could otherwise accumulate. When you hold the pilot control knob in during lighting, you are manually holding the gas valve open while the thermocouple heats up in the pilot flame. Once it is hot enough, it takes over holding the valve open itself. Release the knob too soon, or if the thermocouple is not heating properly, and the valve closes again, extinguishing the pilot.
What is safe to check yourself
Checking whether the pilot is lit, following the exact relighting procedure printed on the unit, and holding the knob for the specified time are generally accepted homeowner-level checks. Adjusting the gas valve itself, replacing a thermocouple, or any work beyond the printed lighting instructions is a job for a licensed gas engineer.
Common causes
| Cause | How likely | Typical sign |
|---|---|---|
| Knob released before thermocouple is fully heated | Very common | Goes out within a few seconds of releasing |
| Thermocouple worn, dirty, or misaligned | Very common | Repeats every time despite holding the knob correctly |
| Draft blowing the pilot out | Common | Coincides with windy conditions, or a nearby vent or door |
| Dirty or partially blocked pilot orifice | Common | Small, weak, or oddly coloured pilot flame |
| Thermocouple connection loose at the gas valve | Occasional | Intermittent, sometimes stays lit longer than other times |
| Failing gas control valve | Uncommon | Persists even after thermocouple attention |
Working through it
- Follow the printed procedure exactly
Locate the label on the unit with the model-specific lighting sequence and hold the knob in for the full time it specifies — this is usually 30 to 60 seconds, longer than most people naturally hold it before giving up.
- Check the pilot flame itself once lit
It should be a steady blue flame, roughly an inch or so tall, and should be enveloping the tip of the thermocouple. A small, weak, yellow, or oddly shaped flame suggests the pilot orifice is dirty or the thermocouple is not positioned correctly in the flame.
- Rule out a draft
Check for anything that could be blowing air across the pilot area — a nearby air vent, an open window, or airflow from an adjacent room, particularly if the unit is in a closet or utility space with air handling nearby.
- Check the thermocouple connection
If you can see where the thermocouple connects to the gas control valve, confirm the fitting looks properly seated rather than loose — but do not disconnect or adjust it yourself beyond visual inspection.
Thermocouples wear out gradually over years of service and are a routine, inexpensive replacement part. If the unit is several years old and this is a new, recurring problem, a worn thermocouple is the most likely explanation and replacing it is a standard, quick job for a gas engineer.
When to call a gas engineer
Call a licensed gas engineer if the pilot will not stay lit after following the correct procedure and holding the knob for the full specified time, if the flame looks weak or oddly coloured, if you suspect the thermocouple needs replacing, or if this is a recurring problem on an older unit. Call your gas utility’s emergency line immediately, rather than a general engineer, if you smell gas at any point.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
