Set Appliance State:
Mains Power Off
Gas On; Pilot On
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Boiler Operating Switch in HW position
Ensure DHW is cool
Ensure DHW is cool
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With the DHW tap fully open measure the DC Voltage across the modureg solenoid (Yellow wires).
The voltage obtained should be approximately 24 volts DC.
The voltage obtained should be approximately 24 volts DC.
Isolate as per TOPs P4
and refit as necessary