Ad Blue or Diesel Exhaust Fluid (DEF) Explained

Published Date: 4th Mar 2016

This fluid has been used on a commercial basis for several years now helping to reduce NOx gases coming from the larger engines on HGV’s, Buses and other large diesel vehicles.

From September 2015 all new vehicles sold in the EU must meet the Euro 6 emission standard, the aim of this is to continually make cars cleaner and reduce the nasty chemicals like nitrogen oxide (NOx) and carbon monoxide (CO) being exhausted into the atmosphere.

This all has a knock on effect for us, as cars get more efficient they will do more mpg and have a lower Co2 output per kilometre which means less money paid out for tax.

Therefore new measures have been developed to reduce the harmful gases released and one of them is called SCR (Selective Catalytic Reduction). This advanced emissions control system uses Ad Blue or Diesel Exhaust Fluid. DEF is water based, biodegradable and non-toxic that is made up of deionised water mixed with urea (found in urine)  

The fluid is stored in a tank separate from the fuel and is injected into the exhaust system after combustion has taken place. The urea based fluid then vaporises when injected into the hot exhaust gases and forms ammonia and carbon dioxide, the NOx gases (produced naturally in the combustion cycle) are reduced into water and nitrogen as a result of the reaction to the ammonia.

So how do I know the Ad Blue fluid needs topping up? And how do I top up the Diesel exhaust fluid?

Don’t worry, the manufacturers have systems in place to tell the driver through the driver information system that the fluid is getting low well in advance with light warnings coming up on the dash giving you plenty of time to top the fluid up.

Always consult your car’s handbook or online handbook as many of them are these days and you’ll either find the top up point located in the boot under a side panel or located next to the fuel filling point.

Where can I buy the Diesel Exhaust Fluid to top up the Ad Blue?

The fluid is readily available at your local dealership or local Halfords and usually weighs in at £1.50 a litre and will come with the filling adapters. Make sure to check you handbook for the capacity of you additive tank so you know how much to buy.