Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Isolate DHW valve underneath appliance
(using mains stopcock is also acceptable)
Refit flow switch from previous test if you have not already done so
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage from pin 5 at connector
X13 to a suitable earth point.
Is the voltage now approximately 4.8V DC?
Note:
This test can be carried out while appliance is doing self check.