Set Appliance State:

Mains Power Off
DHW Demand
DHW set to MAX
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Run the hot tap and allow the boiler to light and establish.
Monitor the appliance for 2-3 minutes.
Does the appliance return to a temperature related fault code within this time period?