Your boiler's pilot light is failing to stay ignited after you try to light it, meaning the main burner cannot start to provide heat.

PILOT WILL NOT STAY LIT WHEN THE GAS CONTROL VALVE KNOB IS RELEASED

What causes this fault?

This fault occurs when the boiler's safety mechanism fails to detect a healthy flame, causing it to shut off the gas supply for safety. The most common reason is a worn-out thermocouple that can no longer generate the small electrical current needed to hold the internal gas valve open. It can also be triggered by a faulty overheat thermostat or a blockage in the pilot injector preventing a consistent flame.