Set Appliance State:

Gas Off
DHW Demand - Ensure Store is not up to Temperature
Isolate Mains Power to appliance as per TOPs P4
Attach FGA + ve pressure connection to gas outlet pressure test point on the gas valve
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Appliance On/Off switch On.
Reset lockout as necessary.
Allow the fan to run while observing the FGA.
As the appliance attempts to ignite the fan speed will increase and decrease 5 times.
While the fan is running does the fan pressure reach -2.5mb or greater (e.g. -3.0mb)?
Notes:
When pressure gauge is removed test point should be tightened and checked for soundness.