Set Appliance State:

CH Demand
Isolate Mains Power to appliance as per TOPs P4
Remove one lead from the modulating coil
Attach U Gauge to burner pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the low rate burner pressure this should be approximately 2.5mb
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.