Set Appliance State:

Mains Power Off
Gas Off
Turn Hot Water Tap On
Boiler ON/OFF switch ON
Ensure no CH demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance if it is in lockout.
Does fan run with a DHW demand returning to fault code?
Fan may run on low speed if DHW demand is removed, only select
fan runs if the fan runs whilst DHW demand is present (LED lit).