Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Ensure radiators are open and water can circulate around the system
Attach temperature clamps to the CH flow pipe and to the internal bypass pipe (both located underneath boiler)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to fire and run for at least 2 minutes.
Measure the temperature of the flow pipe and the bypass pipe.
Is the flow pipe hotter than the by-pass pipe?
Or is the bypass pipe the same temperature or hotter than
the flow pipe, indicating that the internal bypass has opened.