One of the two copies of the key fell and the plastic shell broke. The key was still working and starting the car, but the shell was all craking and wobbling, so I bought a replacement empty shell on Amazon.
I extracted the remote control unit and the key head from the old shell, inserted them into the new shell, and closed it.
Now the key does not start the car anymore. It makes contact, and when I turn it the starter motor spins, but the engine does not start.
Have I missed something? Is there maybe a third component in the original key that I left over? I thought (also based on the replacement shell instructions) that there were only two components to carry over from the old key: the key head and the two-button remote control. But what about the immobilizer/transponder? Is that built into the remote control unit? Or is it a separate item? How do I identify it? I still have the old broken shell.