Set Appliance State:
Mode selector switch to Off/Reset position
Set Multimeter to AC Volts 
Isolate Mains Power to appliance as per TOPs P4
Hot water temperature control set to maximum
Central heating temperature control set to maximum
Gain access to PCB, use insulation mat as required
Hot water temperature control set to maximum
Central heating temperature control set to maximum
Gain access to PCB, use insulation mat as required
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Mode selector switch to CH position.
Test 1:
Measure the voltage across brown and blue wires on connector M11 at the main PCB.
Test 2:
Turn hot water tap ON.
Measure the voltage across black and blue wires on connector M11 at the main PCB.
Test 1:
Measure the voltage across brown and blue wires on connector M11 at the main PCB.
Test 2:
Turn hot water tap ON.
Measure the voltage across black and blue wires on connector M11 at the main PCB.
Isolate as per TOPs P4
and refit as necessary