Andy, the relay link lead was fitted by bmw as a cold start leaner mod.Certain of these cars overfuelled on crank and to get round the prob this kit was fitted to fool the ecu into thinking the engine was warm during crank.It is advisable to refit this and also if the plug you repaired was corroded the cts sensor runs thro this with a brown red and brown orange leads so if these are iffy you will still have probs .Hth Phil