Battery changed now alarm goes off and door ajar.

Are you using the key in the door handle or are you using the remote?

You might have a bad Door Lock Actuator / Module. You need a scan tool to "view" the door lock module operation by accessing the central locking
control module / central electrics module.

With the scan tool you can "see" what caused / triggered the alarm. This helps in verifying whether or not it is a door lock module or not.

Thomas