Lambda sensor was my next question But I didn't want to put it all in the same thread. I have heard mixed reviews about them, from "they're great best tune up tool ever"- to "complete waste of time, too many variables, dont compensate for temperature properly, proper mechanics ones are $5000 for a reason- a $200 O2 sensor & gauge is just a waste of money"

and probably more convincing negative arguements than positive ones