Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage between the black pin marked 15.2
and the yellow pin marked 15.3 on X5 connector at the PCB.
Voltage should be approximately 2.5V DC.
Notes:
This test can be carried out while the boiler is
completing its self check.
At flow rates below 3 litres a minute you may see
the voltage vary between 2V DC and 3V DC.