Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
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 Red pin marked 15.1 on X5 connector at the PCB.
Voltage should be approximately 9V DC.
Note:
This test can be carried out while
the boiler is completing its self check.