The lambda sensor measures continuously the oxygen content of the exhaust gas and provides this information as a control parameter to the engine control unit (ECU).

Optimal air/fuel mixture = complete combustion =>fuel-air ratio of 1/14.5, or Lambda = 1

Walker lambda sensors make the catalytic converter (CAT) work efficiently and harmful gases are kept to a minimum. We need to know the composition of the exhaust gases + the oxygen content.
  =>  Therefore we need the lambda sensor.

You will find the lambda sensor installed upstream of the catalytic converter. More and more cars are equipped with lambda sensors. Since 2000, most vehicles have an additional lambda sensor fitted down-stream of the CAT to check the performance of the CAT itself.