frontera 2198cc sport year2000?

the key fob does not work (not the battery) I can open the door manually but when I start the engine the alarm goes off and flashers, after a few minutes the alarm goes off and the flashers a few minutes after that, the car can be driven ok during all this and it does not start again until you lock the car manually and then restart the car

Alistair Hodge
January 2011