Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach pressure tube to positive connection on FGA
Attach performance tester to gas valve outlet pressure test point
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

The fan pressure should be -1.5mb or greater (e.g. -2.5mb) as the fan runs at low speed.
During the sparking stage of the ignition sequence the pressure should equalise to 0mb with the fan still running.
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.