Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Drain primary circuit of water
Remove flow and return pipes to condensing heat exchanger

Measurement:

Inspect the heat exchanger water ways for restrictions and/or blockages.
Note:
1. To confirm no restrictions and/or blockages are present it may be necessary to remove the heat exchanger from the boiler and pour water through the heat exchanger to confirm it is unrestricted and/or blocked.
2. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.