ecu fiat punto 05 sport active 8 valve. Blue key?

I have just had a genuine fiat ecu fitted by my garage and the engine did not start, my garage has sent the unit back to the fiat dealer and they are sending another. is this a common problem with fiat replacment parts. Why would a fiat replacment fail from new ,registration and chasis number were supplied on ordering, how can this be solved?.
The car showed the usual problems( firing on two cylinders,engine warning light on.This is the reason we had both coil packs and the ecu replaced but the car wioll not start at all.