I assume that the 7,5A fuse that you are removing and replacing is F18, which normally provides a permanent (24/7) supply to the Code Control Unit.
Unfortunately there are no relays that could latch closed, so the most likely causes of failure are thermal with a micro crack in one of the components, or a firmware problem causing a lockup in the software.
I am sure that any simpler explanation would be most welcome.
The external wiring of the CCU, is relatively simple. It consists of a dedicated positive supply from fuse F18, with an earth connection at or close to the ECU. There is a short 2 core screened cable to the aerial on the ignition switch. Outputs are to the code lamp, and to the ECU. Finally there is a connection to the diagnostic socket.
The output or communication to the ECU is routed via connector D004 pin C19, and the earth connection is via D004 pin A2.
I believe that connector D004 is the multiway connector, under the engine bay fuseboard. I have read of corrosion occuring in this area, and the possibility could be worth eliminating.
To access D004, first disconnect the battery negative post. The fuseboard can then be unlatched, and carefully moved aside.