Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Remove electrical 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 the DC voltage between pin 3 (black) on
connector X5 and a suitable earth point.
Expect approximately 5V DC.