Set Appliance State:

Mains Power Off
CH Demand
CH temperature control to max
Ensure flow and return isolation valves are open
ON/OFF switch ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to run on CH demand for a few minutes.
Does the internal by-pass pipe reach a greater temperature than boiler flow pipe ?
Note:
Use a FGA (Flue Gas Analyser) to measure this temperature difference.