Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
CH Temperature Control to MAX
Place temperature probe in flue gas test point, ensure it is fitted securely to prevent any release of flue gases.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary.
Allow boiler to complete its self check.
Monitor the temperature as the appliance operates for heating.
When the boiler shuts down is the flue gas temperature approximately 95°C or higher?