X300 immobiliser

Hi, I have been reading posts on immobiliser subject.
Unfortunately, I am in this situation where the “Check Engine” light is off when I attempt to start the engine and battery was flat for some weeks. The engine does not crank. Key fob works normally.

From what I read, people buy new security & locking module with associated keys to solve the problem. I was wondering if there was a procedure to reset the synchronisation ? It is not clear for me why this module needs to be changed.

There was also the idea to bypass the immobiliser module like for US version of these cars. Has anybody succeeded in doing this modification ?

I must have a drain on the battery that explains why it was flat after some weeks. It can be tricky to solve this drain, so I was thinking to install a circuit breaker to keep battery up. Does anybody have experience with circuit breaker, is there any drawback on security module ?

Thank you. Jonathan.

WHAT CAR do you have??

X300 , 6 cylinders, 4.0 normal aspirated, automatic transmission. It is from 1995 and bought in Italy, so left hand drive.

With the “valet” key, the engine started. So the issue was with the main key that was not recognized (a consequence of flat battery I assumed). I still don’t understand why the valet key is recognized and main key is not.

Sometimes keys are ‘electronically-forgotten’.
When the ‘forgotten’ key is re-configured, ALL keys must be present so keep them all together.

The X300 in North America does not have RFID keys so I am only familiar with the system from 1998 Jaguars onward.

‘Electrickery’ is sometimes a mystery?