Will I get a CEL? This is for down the road, should one of them fail. My Focus is a PZEV so it has 2 catalytic converters. AFAIK, the second o2 sensor (post cats) just measures a change in temperature. So if one cat was still there, I think the sensor would still be happy. Also, I think the PZEV model makes 5 less hp because of that 2nd cat; I know they're quite restrictive. I might even pick up an mpg or two.