Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
CH Temperature Selector to MAX
Attach a temperature clamp to the flow pipe as indicated by the red arrow
Attach U-gauge to burner pressure test point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to run in CH mode.
The boiler should run on CH until it reaches maximum
operating temperature (approximately 75-80°C).
As it approaches maximum temperature it should modulate the burner pressure
to maintain temperature or shut down, without the overheat stat tripping.
Does the appliance behave as described above?
Note:
Overheat indicated by either the manual overheat tripping or if auto resettable overheat is fitted appliance will start sparking then go to lockout. Click HERE for more information.