immobliser fault D9 hdi 99

[shaun406]

i changed my bsi from a donor car the other day from a v6 coupe with the key+chip and ignition surround and wiper stalk as i wanted to trip function on mine which the v6 had and when i first changed it all over i to mine it all worked apart from the mpg was just constantly counting up, it started fine and now all of a sudden the key light is on and it wont start and it says on the mfd ignition fault or sumit what could be causing this.its strange how it worked fine then just stoped