Set Appliance State:
Set Multimeter to DC Volts 
Isolate Mains Power to appliance as per TOPs P4
Expose PCB by removing plastic control box cover.
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in DHW Summer position
Hot Water Temperature Control set to Maximum
Expose PCB by removing plastic control box cover.
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in DHW Summer position
Hot Water Temperature Control set to Maximum
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Measure approximately 5 volts DC across connections
D22 (+ve, White wire) and D18 (-ve, Black wire) on PCB.
D22 (+ve, White wire) and D18 (-ve, Black wire) on PCB.
Isolate as per TOPs P4
and refit as necessary