Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
External controls calling for heat
Attach temperature clamps to the flow and return pipes as indicated by the red arrows
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to fire and run on CH for a few minutes.
Check the temperature differential of the flow and return pipes inside the boiler.
There should be a differential of at least 11°C.
Is the temperature differential correct?