Explain how petrol and diesel cars may be a source of market failure?

As with many questions in economics A-level, I would start my answer with a paragraph explaining how under certain assumptions, an efficient allocation of resources can be achieved through the market mechanism. I would then highlight the assumption used in the model that does not hold in the case of diesel and petrol cars: that there exists no externalities in the consumption or production of the good being traded (and I would ensure that I am clearly defining all technical terms such as externalities). In order to explain this I would use the standard negative consumption externalities diagram, highlighting the area that represents the deadweight loss and what this represents in the standard economic model. Throughout my answer, although not necessary I would try include other relevant pieces of knowledge such as referencing the key economists who contributed to this theory and its application in the real world.