hiya guys bought afr gauge of one of my mates today hoping to fit this in my my99 over this week at some point there is a red blakcand purple wire annd thats it anyone got ideas am i missing a sensor or something?????? im guessing the red and black to be the obvious but what about the purple where does that go ?????

 

What gauge is it? Sounds like you've bought a narrow band gauge so it will need wiring to the signal wire of the standard Lambda sensor. Won't be much good though as it will read rich all the time when on boost. You need a wideband sensor, controller and gauge like this one for it to be accurate.