Please note that EFIE devices A) are not legal for use on public roadways and B) have been found by me and others to be of very little value anyway. ECUs are much too smart for such a simple device. The plans described here are for informational purposes only. I do not use one on my car and I do not sell them. Please do not ask. I will not build one for you.

If a vehicle is equipped with dual o2 on a single cat, you only need to offset the one before the cat. The only time you need dual O2 EFIE is if you have dual cats or dual O2 sensors before the one cat like my honda on the factory header manifold.

But here's a new twist. Newer vehicles are now using a combination of O2 sensors after the cat and wideband A/F sensors before the cat. The voltage curves between them are opposite! The only way to tell for sure what you have is to pierce the wire going to it, rev the engine once warmed up to around 2500 RPM and measure the voltage in operation with a good DMM. O2 sensors will float around 0.5V, A/F sensors around 3.2V

On the O2 sensors, + from the EFIE goes to the ECU, - to the O2 sensor.

On the A/F sensors, - from the EFIE goes to the ECU, + to the O2 sensor.