**The useful distinction is whether the pilot goes out while you’re still holding the lighting control, or only when you release it.** A brief flame alone doesn’t establish that the thermocouple is faulty.

The Honeywell V4600C is a gas valve for a standing pilot with a thermocouple—a sensor heated by the pilot that allows the safety valve to remain 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).

Based on that arrangement, these are the likely diagnostic branches:

- **It stays lit while the control is held, then goes out on release:** first check the boiler’s own lighting instructions, including the specified holding time. Possible faults include insufficient flame heating the thermocouple, a faulty thermocouple or connection, or a defective valve safety unit.
- **It goes out even while the control is held correctly:** pilot gas flow, a restricted pilot injector, or a valve fault needs investigating. Replacing the thermocouple alone may not solve this.
- **It rarely catches at all:** the engineer should also check the igniter and its position relative to the pilot.

Your gas fire working confirms gas reaches the house. Gas at the boiler inlet **doesn’t prove adequate operating pressure or flow through the pilot circuit**.

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

For your own checks, find the **boiler make/model and its lighting instructions**, and note what happened during your previous attempts. Given its unknown service history, 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).