I had the same problem with Renault - changed the battery in my key then it wouldn't work - they told me 'this happens' - new key required me to re mortgage the house as it had to go to 'an approved dealer to be re-programmed' which added to the already outlandish cost - slipped the mechanic a fiver and he showed me how to override the immobiliser - it means I can never rob a bank cos it takes about a minute to do it each time I want to start the car if I remove the key - however if you leave the key in the ignition and don't turn it right off, the car can be started as normal.