Set Appliance State:

Mains Power Off
Gas On; Pilot On
Turn Hot Water Tap On
Selector Switch OFF
Gas Control Knob in Main Burner position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Remove Hot Water demand.
Does the fan continue to run and does the pump start up?
Note:
Do not take sound or vibration as an indication of operation.