Your boiler is failing to keep its small starter flame lit, which prevents the main heating system from firing up.

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 constant flame, causing it to shut off the gas supply for safety. The most common reason is a worn-out sensor called a thermocouple that can no longer signal that the pilot light is safely lit, or a slight blockage in the pilot burner preventing a strong flame. In some cases, it may also be triggered by an internal safety switch that has tripped to prevent the unit from overheating.