How to fix the key remote of my 2001 Chrysler PT Cruiser?

The remote on my key-fob doesn't lock/unlock the doors. Direct use of the key sets the alarm going which gets turned off only when the key is in the ignition and the car is started. I've changed the batteries but nothing. So far I've found a way round it- not ideal, but helps prevent me leaving stuff on the curb, jumping into the car to beat the alarm b4 it sets off! First, I lock it centrally from inside the car, then open my door to exit, 'press'-locking it from inside, then slamming it shut. What can be done?