Set Appliance State:

Mains Power Off
CH Demand
Selector switch set to DHW/CH
CH temperature control set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to ignite and run on central heating operation to a reasonable temperature.
Does the system pressure remain stable or rise excessively.
Note.
With system cool, pressure should be approximately 1 - 1.5 bar.