Central locking not working on key.

You may have to reprogram your key. If you turn the key twice to the unlock position while the key is in the door, it should unlock all the doors.

try this to reprogram your key after battery replacement.
Procedure

1. Switch Ignition ON and leave the key in the ignition.
2. Mechanically lock the vehicle using the Drivers door using a second key.
3. Using the remote control press the Unlock button once, and the horn will sound once.
4. Wait 6 seconds and press the unlock button once to activate the coding
procedure. The remote control will now be programmed.

NOTE : When performing programming on additional remotes the procedure
is the same, except you press the unlock button 2, 3 or 4 times for the
different remote.
For example : If programming 2 remotes press the unlock button twice with
a 1 second interval between each press, repeat for 3 and 4 remotes.

5. Switch Ignition OFF.
6. Remove key from ignition and check for operation.
7. The coding procedure can be repeated up to 4 times for additional remotes.
8. The ignition must be switched OFF between programming each remote
control.


Remote Control Erasing (1997 ON)

Procedure

1. Switch Ignition ON and leave the key in the ignition.
2. Mechanically lock the vehicle using the Drivers door using a second
key.
3. Press the remote control unlock button 5 times in 1 second intervals.
A short horn will sound.
4. Wait 6 seconds and press the unlock button once to activate memory
erase.
5. Switch ignition OFF, all remotes are now erased.

Before you spend bigger amount, check the battery in the key. Usually a CR2032 battery can be find there. (Open the Keyfob by inserting an object into the key ring holder and pulling the top and bottom apart, Seperate the two halfs, Open the top half to expose the battery... )