Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Remove flow switch from appliance but leave connected electrically
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.