Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Gain access to PCB (use Insulation Mat as required)
Pull connector X4 back slightly to expose soldered points on PCB as shown
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

If boiler is in lockout reset it then turn mains power OFF then back ON
so that it starts the appliance self check. (AP shown on display).
With the fan running measure the voltage
between pin 6 (white) and pin 7 (black).
Voltage should be approximately 10V DC.
Note:
Fan must be running to measure correct voltage.