Once the cat is cleared out, these DO work. Here's how:
The FRONT O2 sensors do all of the work in determining the air/fuel ratio. The exhaust passes over them before it gets to the cat. Once the exhaust is 'processed' by the catalytic converter, it is measured again at the REAR O2 sensors. The readings from the rear O2 sensors are calculated against the readings from the front O2 sensors. The difference is an integer, and that integer is used to determine the "efficiency" of the catalytic converter system. Gut the cat, and the readings for both front and rear O2 sensors are the same, as there is no catalytic material to work over the exhaust.
What these "extenders" do is to take the rear O2 sensors out of the exhaust stream, so that no matter what, their readings will be different. This fools your computer into thinking that there are still catalytic converters present, and that they are still doing their job.
