Each button toggles through each of the functions for its associated display so you can see four parameters at a time. There is a menu (maybe settings, sorry I don't have mine with me) that you set your truck's info (engine, trans?, don't remember). Your O2 sensor readings should switch back and forth from a low value (0.2) to a high value (4.5), ther is a lso a parameter tha shouls alternate between lean and rich if the O2 sensor is working correctly. This video walks you through its functions