Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
The burner module assembly should already be removed from the previous test

Observation:

Inspect the heat exchanger flueways for restrictions and / or damage.
Note.
1. Refer to ADAPT regarding any gasket replacement requirements.
2. Only use plastic brushes (not wire) to clean the internal section of the heat exchanger.