If its a E36 BMW then the follow applys Right Number plate lamp, Where the boot lid comes up you will see the wiring is shrink wrapped, undo this and you will find the wires have broken, easy fix, re join the wires! Lambda sensors, replace with new one(dont buy cheap ones), and on the way to the re rest give the car a damn good hard drive!! CATS work better when they are hot, Post up the emmsion readings and i will shed some light on it for you Co2 H Carbons etc