Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
Use Insulation Mat as required
Selector switch to hot water
DHW temperature selector to maximum
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the voltage across the blue and red wires at connector J1 on the main pcb.
Approximately 45V ac should be present.