I think the yellow plastic boxes are the earlier "code" immobiliser from 155 era cars so may not be useable with a "code2" era ECU. There is also a later blue box that I think is CF3 era ECUs.
The immobiliser code boxes pair with the ECU the first time the ECU is powered up and once an ECU has paired with a code box it can't be used with a different code box (unless you can reset the ECU back to factory fresh "virgin" - which I'm trying next).
The code box stores the list of valid keys and when you turn on the ignition the ECU asks the code box if a valid key is present, if the code box says "yes" and the ECU recognises the code box as the one it is paired with it then allows the engine to start.
As I understand it the two ways of adding keys are;
1. Use the 5 digit car security code (on a credit card) and software like AlfaOBD to add a new pre-prepared key to the list stored in the code box (needs OBD port wired to both ECU and immobiliser box - dealer method) or
2. Extract the list of valid keys from the code box memory chip and burn a new blank transponder to match a valid key already in the list (needs eeprom reader and specialised transponder coding hardware & software - locksmith method).
I'm a mechanical engineer, not electronics so this is a bit out of my comfort zone but I'm getting there.
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