Set Appliance State:

Mains Power Off

Observation:

Does the pressure gauge show a pressure of approximately 1 to 1.5bar when cold?
Using the filling loop allow a small amount of water into the system and confirm that the reading on the pressure gauge increases.
Note:
This test is only relevant if the appliance is fitted to a sealed system. If it is fitted to an open vented system select "system pressure correct and pressure increases"