It is a well-known fact that diesel engines emit more harmful gases than petrol engines. Over time, this increased pollution builds up, and can cause serious health problems and even smog and acid rain in densely populated areas. In recent years, laws have been introduced to deal with this increased pollution.
Since 2014, one of the ways car manufacturers have dealt with stricter emissions laws is to introduce a mandatory chemical called AdBlue to diesel vehicles. This urea-rich liquid is a fuel additive which, when added to your engine, is broken down into nitrogen and water vapour (both naturally occurring products), significantly reducing the nasty pollutants emitted by your diesel engine.