Set Appliance State:

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

Observation:

Run the appliance through its ignition cycle repeatedly and listen carefully to it to see how well it is igniting.
The burner should be heard to ignite and then establish (indicated by a flame symbol appearing on the top right of the display) and do so without being excessively noisy.
If the flame symbol does not appear after the burner ignites or only appears intermittently this indicates a rectification issue.
If the flame baffles or is excessively noisy this indicates a combustion issue.