Your boiler has detected an internal electrical or safety fault and has stopped running to prevent damage.

Boiler fails to work. Check: Electrical Plugs (PL1 to PL6) are connected to Circuit Board, Main Supply Voltage is between 196Vac & 253Vac, link plug is fitted (RSL models), or T.T.B. thermostat continuity (CFL models).

What causes this fault?

This fault occurs when the boiler's internal computer stops receiving signals from essential safety components or loses its electrical connection. It is usually caused by a loose wiring plug inside the unit or a component failing to complete an electrical circuit, which forces the boiler to shut down for safety. In some cases, fluctuations in your home's mains power supply can also trigger this red light.