Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove DHW microswitch
Remove CH pump proving microswitch and
fit to the DHW differential valve
Disconnect connector A5 from PCB
Remove CH pump proving microswitch and
fit to the DHW differential valve
Disconnect connector A5 from PCB
Measurement:
Test 1:
Measure for continuity across connection 3 (red wire) and
connection 4 (red wire) at connector A5 on the main PCB.
Expect open circuit.
Hot water tap ON
Test 2:
Repeat test measure for continuity across connection 3 (red wire) and connection 4 (red wire) at connector A5 on the main PCB.
Expect continuity (short circuit).
After test completed refit DHW microswitch.
Measure for continuity across connection 3 (red wire) and
connection 4 (red wire) at connector A5 on the main PCB.
Expect open circuit.
Hot water tap ON
Test 2:
Repeat test measure for continuity across connection 3 (red wire) and connection 4 (red wire) at connector A5 on the main PCB.
Expect continuity (short circuit).
After test completed refit DHW microswitch.
Isolate as per TOPs P4
and refit as necessary