Set Appliance State:
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector Switch in DHW position
DHW Temperature Control to Maximum
Selector Switch in DHW position
DHW Temperature Control to Maximum
Gas On
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow fan to run and ignition sequence to commence.
Measure voltage across the connections on the black solenoid.
Measure voltage across the connections on the black solenoid.
Isolate as per TOPs P4
and refit as necessary