Isolate Mains Power to appliance as per TOPs P4

Observation:

Inspection 1:
Carefully remove sump plate below heat exchanger taking care to accommodate water that may be inside, check that the drain hole to the condensate trap is not blocked.
Inspection 2:
Inspect the heat exchanger flueways for restrictions/blockages
and for good general condition, pour water through and make sure
it drains via the sump and trap.
Note:
Before reassembly check the integrity of all disturbed
gaskets and seals and replace as necessary.