Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Lower PCB
Turn HW stat to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Using telegan temperature probes measure the temperature of the secondary heat exchanger.
Does it get excessively hot or heat up normally?
Check both ends of the heat exchanger in case the temperature is a lot hotter one side than the other. If it is and the temperature is excessive choose that option below.