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