If the electrodes fail to start combustion, a photocell comes into operation.
The photocell consists of a light-sensitive resistor whose signal is monitored by the control box.
Should the boiler fail to ignite after a pre-determined period then the control box shuts off the oil supply and ‘locks out’.
In most boilers ‘lock-out’ is indicated by red or orange neon light and a button is pressed to re-set the burner control system.
