Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Programmer or selector switch set to hot water only
Heating temperature selector to OFF
Hot water temperature selector to maximum
Ensure heat bank is not up to temperature (this can be achieved by running the DHW with the mains power to the appliance on and the gas to the appliance switched off)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the fan run?