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The development of the On-Board Diagnostic Systems (OBD) requires the determination of the critical level of the poisoning of the exhaust aftertreatment components. In order to have an accurate analysis, one way is to separate the effect of the poisoning of the two components: oxygen sensor and three way catalyst [ 1 ]. Our experimentation has been concentrated on poisoning of lambda sensor. An apparatus fed with model gases was realized to evaluate the poisoning of oxygen sensors coming from a fleet test of 6 taxis after a mileage of 100000 km. Our apparatus has been able to differentiate the poisoning level of the sensors by measuring their “switchability”. To confirm these measurements the lambda sensor switchability has been verified with a bench test using the Mercedes- Benz M111E engine. Exposing the aged lambda sensor to the engine exhaust gas, its response was coherent with its behavior on the apparatus fed with model gases.