You may have heard mention of AdBlue, or Diesel Exhaust Fluid (DEF), but don’t know what or why it’s a thing – the simplest explanation is that it helps diesel cars hit ever tightening emission legislation. Not every car has the technology fitted, but as general guide, the newer the diesel, the higher the probability.
AdBlue or DEF is a specially engineered chemical, made up from 67.5% de-ionised water, and 32.5% urea, although this is a synthetic urea (you really don’t want to know the origin of ‘natural’ urea), that gets injected in to the vehicles exhaust gases as part of the Selective Catalytic Reduction (SCR) technology; the result breaks down the harmful mono-nitrogen oxides in to harmless nitrogen and oxygen.
The technology has been around since around 2002, although
it was mainly used on commercial vehicles such as buses and heavy-goods
lorries, but with the introduction of the Euro 6 regulations in 2016,
manufacturers saw it as a way of gaining compliance with the tighter
regulations, although the SCR technology has been used by some brands since
A car fitted with the DEF technology will have a separate tank specifically for the Diesel Exhaust Fluid, depending on the manufacturer, the location for the filling point will vary, as will the tank size – typically, DEF tanks range from 5 litres to 20 litres, and on average, you’ll use a litre of DEF or AdBlue every 600 miles, so it may be possible that you’ve never had to think about the fluid – it all being taken care of in your annual servicing.
But what happens when it’s low?
Any car using this system will give you plenty of warning when the system is getting low, so in theory, you should never actually run out, but if you do ignore the messages to fill up, and the tank finally empties, all that happens is that your car won’t start – you won’t actually cause any mechanical damage.
Some motorists keep a bottle of AdBlue in the boot of their car, for just such an emergency, but given its corrosive nature, we wouldn’t recommend that. Equally, if you’ve opened a bottle to top up the system yourself and not used it all, it has a limited shelf life and will degrade over time, meaning that you’ll need to open another bottle.
Of course, should you get your local dealer to top up your system, you needn’t worry about carrying a bottle of corrosive liquid in your boot, nor wasting a half-empty bottle.