Set Appliance State:

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

Observation:

Allow the appliance to fire and run in DHW mode for a couple of minutes.
Measure the temperature differential between the flow and return pipes.
Expect a temperature differential of 25°C or less.
Note:
Ensure a good contact is made with the flow and return pipes with the temperature clamps to avoid an inaccurate reading.