Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach a temperature clamp to the flow pipe of the primary heat exchanger,
It is not possible to connect a clamp to the return, so a temperature probe will need to be placed on the return pipe of the primary heat exchanger
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 the 4 minute pump purge to complete.
Allow the appliance 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 20°C or less.