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 to the left side of the resistor marked R24 on the PCB.
Expected voltage approximately 5V DC.
Test 2: - Turn Hot Water Tap ON
Measure the voltage between the black wire marked "D" on M6 to the left side of the resistor marked R24 on the PCB.
Voltage should now be approximately 0V DC.
Measure the voltage between the black wire marked "D" on M6 to the left side of the resistor marked R24 on the PCB.
Expected voltage approximately 5V DC.
Test 2: - Turn Hot Water Tap ON
Measure the voltage between the black wire marked "D" on M6 to the left side of the resistor marked R24 on the PCB.
Voltage should now be approximately 0V DC.
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Isolate as per TOPs P4
and refit as necessary