Set Appliance State:

Mains Power Off
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On
Press and hold the Select button for 3 seconds or greater then release to reset the appliance

Observation:

Does the boiler revert to standby?
(Indicated by the time of day being shown)
Does it stay in fault mode?
Note:
Display may change to 3 dashes alternating with the time, this means that the PCB has reset but the temperature is still above 50 °C. The boiler will not fire until the temperature drops below this temp.
There is no pump overrun on this lockout, only the fan overrun. Therefore the boiler may take a while to cool down to below 50°C.