Set Appliance State:

Mains Power Off
CH Demand
Selector switch to DHW and CH position
CH temperature selector to maximum
DHW temperature selector to minimum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run and burner to ignite.
Does CH system operate satisfactorily?
Note:
Overheat indicated by either the manual overheat tripping or if auto resettable overheat is fitted boiler will start sparking then go to lockout.