Isolate Mains Power to appliance as per TOPs P4
Note
This test is relevant only if the appliance is connected to a sealed heating system. Enter system pressure satisfactory if appliance is connected to an open vented system and read diagnostic text.

Observation:

Check for correct system pressure on system pressure gauge.
This is nominally 0.7-1.0 bar however system may require different pressures and this should be investigated as necessary i.e. BS 5449.