Set Appliance State:

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

Observation:

Observe the system pressure gauge, while allowing the burner to fire and the CH system to heat.
Does the system pressure remain constant between 1 and 1.5 bar?