A diagnostic apparatus applied to internal combustion engine exhaust gas sensors employing electrochemical polarographic principles indicates sensor operation relative to a current limiting operating region in which a sensor output signal indicates {HEAD ~ oxygen concentration} in {TAIL ~ the exhaust gas} 
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A diagnostic apparatus applied to internal combustion engine exhaust gas sensors employing electrochemical polarographic principles indicates sensor operation relative to a current limiting operating region in which a sensor output signal indicates { HEAD ~ oxygen concentration } in { TAIL ~ the exhaust gas }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL