Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Boiler Switch in ON position
Central Heating switch in OFF (central) position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the mains voltage across connector PL3 connection 3 (brown) and connection 5 (blue) wires of GV1 & 2 at full sequence board.
Note:
Boiler may go to lockout, switch off power and press reset button to cancel lockout.
Voltage to the gas valves ceases when lockout occurs.