An MIL Eliminator, also known as a Malfunction Indicator Light eliminator or oxygen sensor simulator, is a small wiring harness with a plug on each end that plugs in between your oxygen sensor and body wiring harness. It tricks your Mustang into thinking your oxygen sensors are reading correct oxygen levels after installing a catalytic converter-less midpipe.
If you are installing an off-road h-pipe or x-pipe (no catalytic converters), your factory oxygen sensors will read a higher level of emissions which will trigger a check engine light or malfunction indicator light. To correct this, you can unplug your O2 sensor from the car (while leaving it screwed into the midpipe) and plug the MIL eliminator into the sensor and the body harness. This will turn the check engine light off harmlessly and allow for normal driving capabilities.