Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Unplug connector CN4

Measurement:

HW Tap Off:
Check for open circuit resistance across the DHW Flow Switch (Red wires)
on connector block CN4.
HW Tap On:
Check for short circuit resistance across the DHW Flow Switch (Red wires)
on connector block CN4.
Note: Test should be carried out at disconnected connector.