i think white wire of the o2 sensor. sold mine a while ago. its really a useless guage unless your running one with a wideband o2 sensor. its more for show. i got one cuz at the time i was running nitrous and thought it'd come in handy but when i sprayed it would just go to full rich. the only time it has really done anything useful was when i messed up on the nitrous jets and it went full lean i shutdown immediately and looked on my nitrous jet and there wasnt one in there!