Set Appliance State:

Mains Power Off
Ensure appliance is cool
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset the boiler as necessary.
Does the appliance now complete its self check and go to standby mode (status 203), or does it return to fault code?
Boiler resets and enters standby - retest appliance
Boiler will not reset or resets and returns to fault code