cliffyk

What you are asking about is not really a "mod" in the usual sense, as there is no performance benefit associated with it. It is a "fix", mostly used to address a problem that attends the installation of a so-called "off-road" mid-pipe (one having no catalytic converters).

Our cars have four O2 sensors--two on each side--one upstream of the catalytic converters, and one downstream (up and downstream are referenced to the direction of the exhaust flow).

The upstream (front) sensors are used to provide feedback to the PCM during closed loop operation, allowing the PCM to dynamically adjust the amount of fuel injected and keep the average air/fuel ratio at or about 14.64:1. This is called the stoichiometric ratio, and is the mixture of air and fuel that burns completely.

The front sensors cannot be removed, or interfered with, without making the engine run like crap.

The downstream (rear) sensors are used only to monitor the efficiency of the catalytic converters. This can be done because the converters store oxygen in normal operation, causing the signal from the rear sensors to be lower in voltage than the front sensors, and to switch more slowly. The PCM monitors the rear sensors, and compares their output with that of the front sensors, to measure the performance of the catalytic converters.

Obviously, when an off-road mid-pipe is installed, the absent catalytic converters are quite genuinely "not working". This means the rear O2 sensor signal will be identical to that of the front sensor, and the PCM will barf out a Diagnostic Trouble Code (DTC) and light the Malfunction Indicator Lamp (MIL), which is actually the Service Engine Soon (SES) indicator on our cars).

There are two solutions to this:

One is to disable the catalytic converter monitoring in the tune, generally referred to as "turning off the rear sensors". This method allows the rear sensors to be physically removed from the exhaust system, unplugged from the vehicle's wiring harness, and thrown in the trash if so desired.

The other solution is to insert an Resistor Capacitor (RC) filter in series with the rear sensors. These passive electronic devices are generally called MIL Eliminators, which makes sense as that is what they do,

The filter is configured to have a -3dB "down" point around 0.15Hz. When the normal 1.0Hz to 5.0Hz signal from the rear sensors (with no cats) is passed through this filter the signal is attenuated by -15dB to -30dB, and phase shifted a bit--this tricks the PCM into believing the catalytic converters are installed and functioning.

Obviously using this method requires that the rears sensors be installed and working properly, or there would be no signal to modify.

You can read more about MIL Eliminators here, and more about how narrow band Platinum/Zirconia (PZ) O2 sensors work here.