Accenta g3 alarm - warning Beep for service (how to switch off)?

Warning Beep comes on every 5 mins, read instruction manual and this indicates that alarm needs a service.

Tried to re-set this but engineers code 9999 has been changed by engineer (cant get hold of engineer) is there anyway to re-set/stop alarm from beeping erery 5 mins?

How do you re-set engineers code back to 9999?