Many new diesel cars use a fluid called AdBlue. If your car uses AdBlue, you'll probably need to top it up at least once between services. AdBlue is an exhaust fluid, not a fuel additive. It's stored in a separate reservoir and is topped up via a (usually) blue filler cap located either next to your fuel filler, in the boot or under the bonnet. AdBlue is a colourless, non-toxic mixture of urea and de-ionised water. It’s not actually blue at all. Tiny amounts of AdBlue are injected into the flow of exhaust gases. At high temperatures AdBlue turns to ammonia and carbon dioxide. Inside the SCR (Selective Catalytic Reduction) catalyst, harmful nitrogen oxide in the exhaust reacts with the ammonia and is transformed to harmless nitrogen and water. Similar technology has been used effectively for years in buses and heavy lorries.