an apparatus for determining the condition of engine lubricating oil includes a sensor having a plurality of spaced apart electrode pairs on a nonconductive polymer film . the sensor is operatively associated with an oil filter , which provides retro - fit capability and easy access for servicing and replacement . the multi - electrode pair design averages signal output to reduce operational electromagnetic interference noise . a forcing - function waveform reactive circuit is applied to the sensor input electrode as a common voltage potential . the output current from the sensor electrodes is then relayed to a current follower amplifier , which converts the output current into an equivalent voltage for comparison with predetermined values . based on the voltage values , the on - board sensing system will determine the oil &# 39 ; s condition , and will trigger a trouble code if the equivalent voltage falls within a predetermined range .