Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Remove the connector from the DHW flow sensor and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure for DC volts across pin 3 (yellow lead) connector X2 and a suitable earth connection.
Expected voltage approximately 4.8 Vdc.
Note.
Pins are numbered on the connectors.