Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
Loosen the outlet test point screw (P2) and attach U-gauge to the test point.
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With the hot water tap running.
Does the burner light and go to maximum burner pressure (approximately 8mb) after a few seconds?
Note:
When the U-gauge is removed, ensure the pressure test point is checked for soundness.