Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamps as indicated by the red arrows
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan purge to complete (2 minutes).
Turn hot water tap on.
Allow the burner to ignite and the boiler to start to heat the DHW, measure the temperature differential between the flow and return pipes to the primary heat exchanger.
Expect a temperature differential of approximately 25°C or less.
Is the temperature differential as expected?