I found this great deal on a 97 Explorer (150K mi) that I'll use just to get to and from work 15 mi. away. The problem is this: the guy who's selling it to me forgot to mention that it failed emissions twice in March of '06 before he took it off the road (he says it was a 3rd vehicle anyway, and he was saving it for his daughter, but now she doesn't want it). He thinks that it's probably nothing more than a sensor, but I'm a little concerned. I've worked on cars, so that doesn't scare me. My question is: in everyone's experience, how often is this a huge engine problem, and how often is it just an O2 sensor or a cat?