Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Function selector in standby position
Attach pressure gauge to outlet test point
Hot water tap On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Turn function selector to DHW position.
Allow fan to start.
Does pressure gauge show a pressure of approx 2.5mb;
then drop to approx zero after around 5 seconds;
then after around a further 4 seconds go to 3 mb (see animation)?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.