Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U-gauge to burner pressure test point
Selector Switch in Hot Water Position
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Approximately 2 seconds after the fan begins to run, does the burner pressure increase to approximately 5mb for 8 seconds?
Note:
When "U" gauge is removed test point should be
tightened and checked for gas tightness.