Set Appliance State:

Mains Power Off
CH Demand
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the aid of a FGA, confirm the integrity of the boiler's inner combustion chamber seals.
Areas to check should include:
Above the heat exchanger near the flue outlet.
The inner combustion chamber door.
The sight glass and electrode assembly.
The lower sump connection.
The condensate trap.
Do performance tester readings indicate one or more of the seals has failed?
Note:
If in doubt, isolate the boiler and visually inspect the seals as necessary.