The first two oxygen sensors what role

Some cars are not there two oxygen sensors do. Those first two loaded catalytic oxygen sensor behind the play what role?For the final monitoring of pollutants in the exhaust gas is transformed completely. This is EOBD need.Generally, only need the first catalytic oxygen Crankshaft Position Sensor emissions requirements, but before the catalytic oxygen and aging , causing excessive emissions, after the passage of oxygen contaminant monitoring, one can know before oxygen and ternary catalytic aging, fine-tuning the control parameters, on the other hand can be applied to certain control strategy to eliminate its aging. Of course, if aging is too severe, after oxygen will send a signal to the ECU, the alarm!