Pektron key fob battery change; immobilizer not recognized automatically anymore

I recently changed the battery of the key fob of our MGTF (2005). Since then, when I try to start the car the fob is not recognized anymore and I have to press the unlock button a couple of times before I can start the car.
With the second (spare) fob this is not the case and the car starts immediately.

The fob with the new battery works just fine for locking and unlocking the car. When I start the car immediately after unlocking; I don’t have to push the unlock button

As mentioned on the website, factory key fobs have an auto-disarm feature such that if the fob is inside the car then the presence of the RFID chip in the fob alone will allow the car to be started. But it seems this feature got lost upon the battery change.

I searched on the internet and found a procedure whereby you have to lock the car, press the lock button 5 times and that would synchronize the fob. Unfortunately, it doesn’t solve the issue with starting the car. I also searched this forum, but couldn’t find an answer.

Is it possible to restore this function with Pscan ? Is this done via the “initialise the fobs” button ? Do I then also need to initialise the second (working) fob ? And what are the risks ?
Thanks in advance for some guidance !

There is no RFID chip in the fob. There is electric ring around the ignition key barrel which induces the fob to emit the signal of disarming the immobilisation while fob is in close proximity to the ignition barell.

Inncase of Pekfron fob there is no process of 5 pressing the close button on the fob. This was done for Lucas system in order for the fob to be resynced when battery is changed. However when you change the battery in the Pektron fob there is no such proces as resyncing.

Since you say that you need to press the fob few times to remove engine immobilisation I would look into changed battery, its validity and comtact, because to me this seems like bad battery or poor,contact in the fob.