EFIE Circuit

In the past, fuel savers would not work when applied to fuel systems with oxygen sensor feedback circuits. These systems were designed to prevent efficient combustion!
Modern fuel systems use an oxygen sensor to maintain a constant air-fuel ratio of the engine. Increasing the combustion efficiency of the engine increases the percentage of oxygen in the exhaust because the engine uses less fuel for the same volume of air and more oxygen is free because the engine produces less carbon monoxide and oxides of Nitrogen.
The increased oxygen content in the exhaust is read by the computer to be a lean mixture in the engine. As a result, the computer then adds extra fuel to bring the pollution back to normal. In other words, when the computer sees extra oxygen in the exhaust, it sends enough fuel to maintain a 14.7 parts air to 1 part fuel ratio. When the computer sees less oxygen in the exhaust, it backs off on the fuel to maintain a 14.7 to 1 fuel ratio. The EFIE's function is to modify the oxygen sensor's output-signal by adding a floating voltage; so the computer will not see the extra oxygen and fight your fuel saver by adding extra fuel.