Your boiler's pilot light has gone out and will not stay lit, meaning the system cannot fire up to provide heating or hot water.

Failure of thermocouple, power unit or latching mechanism

What causes this fault?

This fault occurs when the boiler's safety system can no longer detect the small permanent flame used to light the main burner. The most common reason is that the heat-sensing probe has worn out over time or the internal magnetic switch within the gas valve has failed, causing the gas supply to shut off for safety. Without this pilot light being confirmed as active, the boiler is prevented from firing up entirely.