AdBlue
AdBlue is a urea-based fluid sprayed into the exhaust of modern diesel cars to neutralise harmful nitrogen oxides — it’s how diesels meet Euro 6. The car drinks it slowly (a tank typically lasts 5,000–10,000 miles) and warns you well before it runs out; ignore the warnings and the car eventually won’t start. Topping up costs a few pounds at a fuel station or under £20 for a 10-litre tub to do it yourself.
Example
Most Euro 6 diesels need an AdBlue top-up roughly once or twice between services — budget around £20–£40 a year.