Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove connector X3 from the PCB
Disconnect connector from the flow sensor / overheat stat
Disconnect connector from the flow sensor / overheat stat
Measurement:
Test 1.
Measure the resistance between pins 7 (yellow) and 19 (yellow) on connector X3 to the corresponding yellow wires on flow sensor / overheat harness connector.
Test 2.
Measure the resistance between pins 8 (grey) and 20 (grey) on connector X3 to the corresponding grey wires on flow sensor / overheat harness connector.
Expect continuity / zero resistance on all tests.
Measure the resistance between pins 7 (yellow) and 19 (yellow) on connector X3 to the corresponding yellow wires on flow sensor / overheat harness connector.
Test 2.
Measure the resistance between pins 8 (grey) and 20 (grey) on connector X3 to the corresponding grey wires on flow sensor / overheat harness connector.
Expect continuity / zero resistance on all tests.
Isolate as per TOPs P4
and refit as necessary