I have a 2.0 litre petrol Toyota Avensis on a 54 plate. Recently the engine management light came on and I duly took it in to Toyota to investigate. They diagnosed a faulty lambda sensor and replaced it at a cost of around £200. They comfortingly assured me that the car has 4 such sensors and when one goes the rest tend to follow one by one. I can therefore anticipate the pleasure of forking out a further £600 in the near future as they fail in sequence. What is the function of these sensors, how important are they, and could there be any unpleasant consequences of ignoring the failure of them? I am thinking really in terms of increased emissions, or increased fuel consumption, or possibly catalytic converter damage.