Whether the pilot goes out while you’re holding the lighting control or only after you release it helps narrow down the fault. A flame that burns briefly doesn’t, by itself, show that the thermocouple is faulty.

The Honeywell V4600C is a gas valve for a standing pilot with a thermocouple. The pilot heats this sensor, which allows the safety valve to stay open. [Manufacturer’s description](https://homecomfort.resideo.com/sites/Cestina/cs-CZ/Pages/Product.aspx?cat=HW-TOEM&catpath=1.5.1.2.1&pid=V4600C1193%28HW%29).

The likely causes depend on what happened:

- If the pilot stays lit while you hold the control but goes out when you release it, first check the boiler’s lighting instructions, including how long to hold the control. Possible faults include a flame that doesn’t heat the thermocouple enough, a faulty thermocouple or connection, or a defective valve safety unit.
- If it goes out while you’re still holding the control correctly, an engineer needs to investigate the pilot gas flow, a possible restriction in the pilot injector, or a valve fault. Replacing the thermocouple alone may not solve it.
- If it rarely catches at all, the engineer should also check the igniter and its position relative to the pilot.

Your gas fire working confirms that gas reaches the house. Gas at the boiler inlet doesn’t establish that the operating pressure is adequate or that gas flows properly through the pilot circuit.

If checking the inlet pressure point involved loosening or removing its screw, stop further lighting attempts. Have a Gas Safe registered engineer confirm that it is sealed and leak-tight. Don’t open other gas connections, adjust the valve, clean the injector yourself, or bypass the safety device.

Find the boiler’s make, model and lighting instructions, and note what happened during your previous attempts. With its service history unknown, arrange a service and safety check before using it. Ask the engineer to check the pilot, thermocouple circuit, gas pressures, ventilation and flue. [HSE guidance](https://www.hse.gov.uk/gas/domestic/faqs.htm).

If you smell gas, avoid flames and electrical switches, ventilate, turn off the gas at the meter if safe, and call 0800 111 999 in Great Britain or 0800 002 001 in Northern Ireland. [Gas Safe emergency guidance](https://www.gassaferegister.co.uk/media/3570/32492_gsr_lit_guidetosafety_a5_aw1_l6p12-21v1-2.pdf).