Citroen Xsara passenger side front door fault

When the front left door is shut it keeps saying on the computer dash that its still open and keeps beeping to tell me this.but goes off if I lock it from the inside while driving it otherwise it would drive me mad.and when I get out and try to lock it it won't lock as the central locking thinks the door is still open its driving me nuts trying to work it out.any advice would be good.thanks