The only thing that should cause a CEL with regards to the O2 sensors is not running any cats at all, as the sensors then think there's something wrong as the emissions are miles too high (which, of course, they will be!). The O2 spacers lift the sensors out of the gas flow, so that they don't see as much crap in the exhaust as they would do normally so think that the cats are still there. It's not a perfect science, but for 99% of the time they work very well.