Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Selector switch to Hot Water
Appliance DHW thermostat set to MAX
Attach temperature clamps to the flow and return pipes as indicated by the red arrows
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler and DHW to heat to its maximum approx. 65-75°C on the flow pipe.
Does the boiler have a temperature differential of approximately 20°C or less?
Does the fault code reappear?