You have to keep both o2 sensors, and they both have to be reading properly. As already stated the best way to do that is either resistors soldered in line, or a simulator. If you are missing an O2, you will have a CEL. If you use both O2's unmoddified, even if the secondary is after the cat you WILL get a CEL becuase the second o2 is reading the effectivness if the cat. If it reads out of range (this case too rich) that triggers a CEL for a bad catalytic converter, or secondary O2 voltage too high/low.