I see these advertized as a performace interface when adding headers to a pre 96 ODB II car. My neighbor with a Honda, just added headers and an exhaust and an MIL eliminator was included in the kit. What does it do? Are these devices fooling the O2 sensor? Can they be modified for FE?
They are to trick your downstream O2 sensors into believe a cat is still in place. Downstream O2 sensors are only for testing for catalyst efficiency, not for controlling your mixture. Of course, an out of specification reading will cause your car to run in limp mode, which will reduce fuel economy.
Unless you are eliminating the catalytic converters, MIL eliminators will have no effect on vehicle performance, they are basically just to prevent your car from having a MIL (Malfunction Indicator Light, or CEL Check Engine Light) and from running in limp mode when your computer things your vehicle's emissions system is broken.