Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove condensate trap
Remove the sump cover at the bottom of the primary heat exchanger
The sump should be empty if the trap and pipe are clear but it is advisable to take precautions to prevent water spillage

Observation:

Visually inspect the sump, it should be clear of debris and water, also check the sump hole at the back of the heat exchanger is clear.
Is the sump and hole clear?