Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Turn Hot Water Tap On
DHW Inlet valve Off
Disconnect connector plug marked ST 16 from the PCB

Measurement:

Test 1
Check for resistance across the connections 19 and 20 (grey wires) on the disconnected connector.
Resistance should be open circuit.
Test 2
Turn on the DHW inlet valve (hot water tap will still be open).
Repeat measurement across connections 19 and 20 (grey wires) on the disconnected connector.

Resistance should be short circuit.