What does the lambada sensor do?

the lambda sensor senses how much unburnt fuel is coming through in the exhaust gases so that the ECU can regulate it and preserve the correct emissions, usually if the lambda sensor is faulty your EML "Engine Managment Light" will illuminate when the car is under load, eg, on a motorway, also, with a faulty lambda sensor there is a strong possibilty of failing the MOT test and also damaging your catalytic converter