Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamps to the flow and return pipes of
the primary heat exchanger (as indicated by the red arrows)
Note:
Ensure a good connection is made with the flow & return pipes
or misdiagnosis will occur
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to operate for hot water for 2 - 3 minutes.
Monitor the temperature differential across the flow and return pipes.
Expected a temperature differential of approximately 22°C or less.