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
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
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.