I have replaced the battery in the key but the central locking does not work. I can lock the car and deadlock it by using the key in the door lock. I do not know how to locate the remote sensor in the car which I feel might be the cause of the problem.

Ian Fulcher, April 2007

I had the same problem with a Vauxhall.
Try inserting the key in the ignition, turn the key but DO NOT start the engine. Press the lock button on the key within 30 seconds. THe locks will operate. press the unlock button and the locks should unlock. your keys EPROM should now be reprogrammed to your cars immobiliser.
Failing this it could be that during changing the battery a contact within the key has had a dry solder joint, usually indicated by the red LED not working.

Fat Tintin, April 2007