For use on OBD II cars that run 2 Oxygen Sensors.
OBD II cars use two o2 sensors to detect the differences between 1st and 2nd o2 sensor. If it detects the same reading from both sensors, usually because a person is not using a catalytic converter, then the ECU will believe that the car has a faulty cat. This will send out a signal to dash and cause an error code stored in the ECU.
This unit will allow the O2 sensor to sit farther back and not "stick its head out" into the exhaust stream. Basically this allows the sensor to pick up less exhuast readings and allow the difference needed between the two o2 sensors.
This can fix some o2 related problems with OBDII cars, but is not guaranteed to fix basic problems. ( Bad o2 sensors, etc.)