alfa 156 keyfault - immobiliser not working
I have a 2004 156jts veloce. My key was a bit tatty and the eye for attaching it to the key ring had broken off. I had a be and new blank key that I had bought for a previous 156 I owned, so decided to change the case over. I dismantled the keys and swapped the blade and the pcb from my car into the case from the new key. So all I changed was the black plastic case. Everything else from my car key is the same.
When I tried to use the key, the central locking and alarm all work, but the car wont start. The code light is on and stays on when I try to start the car. Why is this? Did I leave the battery out of the key for too long?
Surely if all I changed was the plastic case, the immobiliser chip in the key would still work?
Any ideas would be gratefully received.