Imoberlizer comes on after driving for 15miles?

I have a 2002 18 Zavara elagence I have replaced the sencer below the cat and put a new cat on the car but the light keepes coming on just the same. At this moment in time I am not loosing any power the engine as been checked by a garage who does electronic conputer tests and cannot find any proplems the engine and is runing fine. The problem started when I herd a noice in the egsaurst when the noise had stoped the light came on. Could anyone give me advice on how to get the light of I have been told to run the car as the conputer does not reconize the new sencer and cat I would be grateful of any help that can be give or were cane take car. Ilive in Dewsbury West Yorkshire