Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature probe to pipe as indicated by the red arrow
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Set boiler temperature control for CH to MAX.
Run the appliance and allow the system to heat.
The boiler should in CH mode until it reaches maximum temperature of approximately 88°C, it should then modulate down or shut down without an overheat or circulation error occurring - this is correct operation.
Does the boiler operate correctly?
Note:
To speed up the process the heating load can be reduced by turning off some radiators. If overheat occurs isolate appliance immediately.