Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamps to flow & return
pipes as indicated by the red arrows
Ensure a good connection is made, it may be easier to use the probe style temperature sensor on the return pipe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Cancel purge cycle & if required, reset boiler.
Allow the burner to ignite and appliance to run for DHW.
With the appliance still running for DHW measure the temperature differential.
Expect a differential of approximately 25°C or less.