How can I fix it?

Did you program the remote control after replacing the battery? On our 206cc the procedure is as follows:

Switch off the ignition.
Switch the ignition on again.
Press button A (the larger one which locks the car).
Switch off the ignition and remove the remote control key from the ignition lock. The remote control key should then be working again.

Good luck - let us know how you get on.

ChicksFan
September 2011