Set Appliance State:

Gas Off
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 DHW Flow Switch (Red wires) on connector block.
HW Tap On.
Check for short circuit resistance across DHW Flow Switch (Red wires) on connector block.
Replace connector CN4.
Note:
This test should be carried out on the disconnected connector not on the PCB.