Set Appliance State:

Mains Power Off
Selector Switch to Hot Water and Heating
Ensure there are NO demands
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if it is in lockout.
(make a note of code if you want to use fault code diagnostics)
The appliance should enter standby and the fan should not be running.
The demand LED should not be illuminated.