Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
DHW Demand
DHW temperature set to MAX
Attach pressure tube to positive connection on performance tester
Attach performance tester to gas valve outlet pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow appliance to go through it's ignition sequence.
As the fan starts the pressure should be approximately - 4.0 to - 4.5mb, after 7 seconds this should reduce to 0.00 mb (gas valve opens).
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.