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)
Disconnect wires to gas valve modureg
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to ignite and establish.
Measure the voltage between pins 3
(yellow) and 4 (grey) at connector X2.
Voltage should be approximately 25V DC or greater.
Note:
Remember if a white PCB is fitted X2
is in a different position.