Set Appliance State:

CH Demand
CH temperature set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite and boiler to run in central heating mode, this can take a further 2 minutes from when the demand was recognised on the previous test.
(d1 or d2 may be displayed during this time these are not fault codes)
Does the fault code reoccur? Are there signs of poor circulation? (e.g. rapid boiler temperature rise, excessive noise)