Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Test 1: - Ensure Hot Water Taps OFF
Measure the voltage between the black wire marked "D" on M6 at the main PCB to pin 9 (purple) at the fan control board.
Expected voltage approximately 5V DC.
Test 2: - Turn Hot Water Tap ON
Measure the voltage between the black wire marked "D" on M6 at the main PCB to pin 9 (purple) at the fan control board.
Voltage should now be approximately 0V DC.
Measure the voltage between the black wire marked "D" on M6 at the main PCB to pin 9 (purple) at the fan control board.
Expected voltage approximately 5V DC.
Test 2: - Turn Hot Water Tap ON
Measure the voltage between the black wire marked "D" on M6 at the main PCB to pin 9 (purple) at the fan control board.
Voltage should now be approximately 0V DC.
Isolate as per TOPs P4
and refit as necessary