Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Ensure rads are open and water can circulate around the system
Use an FGA temperature probe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to fire and run for CH.
Measure the temperature of the CH flow pipe and internal by pass pipe.
Is the internal bypass pipe cooler than the flow pipe?
Or is the internal bypass pipe the same temperature or
greater than the flow pipe, signifying the auto bypass
valve has opened?