Set Appliance State:

Mains Power Off
CH Demand
Selector switch to DHW and CH position
CH temperature selector to maximum
DHW Preheat selector to OFF position
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow burner to ignite and system to heat.
Observe system pressure gauge.
Does the system pressure stay constant?
Note:
System pressure should be approximately 1-2 bar.