Can't speak for him but I'm assuming when he read the directions for installation that he mixed up the stock air filter gauge with the sensor? Like I said though it's just an assumptionIf it's just unplugged, your "mechanic" should have caught that right off. Lol
Check your stock air box for a temp sensor held in by a rubber grommet, and a loose plug with 2 wires near the filter. It needs to be plugged in.