Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain boiler primary circuit

Observation:

Undo the top connection running to the condensing heat exchanger.
Inspect condensing heat exchanger waterways for blockages or restrictions, use a mirror & torch as necessary.
Note:
If there are signs of blockages / restrictions then condensing heat exchanger will need to be fully removed to properly inspect & clean.