Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Reconnect connector X2 at the PCB if it has not already been reconnected
Disconnect connector from the DHW flow switch
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Are any of the three left hand LEDs now illuminated?
A fault code may be displayed.