Set Appliance State:

Mains Power Off
Gas Off
Turn Hot Water Tap On
DHW temperature set to MAX
Ensure boiler is cool
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if required (rotate selector switch to
'O' position, then rotate back to DHW/CH position).
Allow self-check to complete.
Does the fan run?
Note:
When mains power is reinstated, at first demand the fan may run for a short period of time, stop, then restart, this is a normal function.