Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect open therm device, connections are 1 & 2 of terminal block & make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to complete its self-check.
If fault code is displayed, reset boiler.
Does the display show the boiler flow temperature (fault code has reset)?
Note:
If the boiler fault code changes from the original to a different fault code, then CLICK HERE.