How To Tell If Your Car Needs AdBlue/DEF


This diesel exhaust fluid (DEF) might just be the secret to keeping your engine in tip-top shape

Read in this article:

What AdBlue Does and Why You Need it

Not many car owners in America need to know about AdBlue, since it is only relevant to a niche market: diesel-engined cars and trucks, which are not overly popular in the United States. Essentially, AdBlue is an additive that can reduce nitrogen oxide (NOx) emissions, reducing pollution and helping drivers meet increasingly stringent emission regulations. It does this by combining with exhaust emissions, breaking down the NOx into nitrogen and water, which are both non-toxic. In the US, this product is referred to as diesel exhaust fluid (DEF).

So, what is AdBlue/DEF actually made of, and how does it work? The chemical composition is 32.5% urea - CO(NH2)2 - and 67.5% deionized water. It is used in vehicles with selective catalytic reduction (SCR) technology that have a separate tank to house the fluid. It gets heated in the exhaust and the urea breaks down into ammonia (NH3) and carbon dioxide (CO2). But instead of creating two different harmful greenhouse gases, the nitrogen ions from the ammonia combine with those of the NOx, while the hydrogen ions combine with the oxygen.

AdBlue System Mercedes-Benz
Catalyst Mercedes-Benz

Pros and Cons

The benefits and downsides of using this product include:

  • Reduced emissions: Naturally, the main benefit of AdBlue fluid is that it reduces greenhouse gas emissions.
  • Fuel economy: While only marginal, the blue diesel additive can improve your mileage figures, which helps to offset its cost.
  • Optimized performance: One of the best AdBlue benefits is improved horsepower and engine tuning for more torque.
  • Less engine wear: By keeping the internal components of your vehicle cleaner, AdBlue can actually reduce engine wear and tear.
  • Price - It is, unfortunately, just another added cost when it comes to car maintenance.
  • Corrosion - There is a minor risk that DEF can corrode low-quality car parts.
  • Freezing - AdBlue freezes at 12.2 degrees Fahrenheit, so if you live in a cold area, you will need to add antifreeze to the tank.
  • Inconvenience - Every so often, you'll need to top up the DEF tank, and forgetting can cause problems.

Not All Cars Require DEF

Nitrogen oxide is not your average car emission - only diesel engines like those found in the VW Passat TDI have this problem, and only more modern variants are able to make use of this handy substance. They have a special tank and pump for it, as well as an AdBlue warning light to let you know when it is running empty and needs to be topped up. Just a few of the cars which use AdBlue in the USA include the diesel models of the :

Most fleet managers prefer diesel-powered vans for their better fuel economy, so DEF is essential in newer models. The same is true for Class 8 semi trucks, which are still used to move cargo around the US.

7 Outstanding Rotary Engine Swaps
7 Outstanding Rotary Engine Swaps
Cars You Wouldn't Expect To Share A Platform
Cars You Wouldn't Expect To Share A Platform
Bluetec Engine Mercedes-Benz
Filler Neck for AdBlue Fluid Mercedes-Benz

How to add AdBlue/DEF and how often

Modern cars equipped with an SCR system have a warning light that notifies you when the DEF needs topping up. How long AdBlue or DEF lasts varies between different models and driving habits, so there is no standard schedule as with other car maintenance. How to add AdBlue is pretty simple thanks to manufacturers being mindful of the new emission regulations. There is usually a filler spout beside the fuel cap on most trucks.

Refueling AdBlue/DEF Mercedes-Benz


What does AdBlue do and what happens if it runs out?

Should the DEF run out, your car will shift into 'limp mode', which hamstrings your speed in an effort to reduce emissions. Don't panic and stop the car, since it may not start again until the AdBlue has been refilled. Unless you have a bottle of DEF on you, the best thing to do is find the nearest gas filling station.

Does my car need AdBlue and can I add it myself?

If you are driving a newer diesel vehicle, then the chances are you do need AdBlue. Most models produced in the US after 2015 make use of the new SCR technology to reduce emissions. Luckily, it is not hard to maintain DEF levels, and you can easily add it yourself using the filler spouts. If you are unsure where to find them, consult your user manual.

What happens if I put AdBlue in the diesel tank?

Accidentally mixing AdBlue fluid with your fuel can cause problems. You will need to get the tank completely drained by calling your repair services, or even roadside assistance. If you were to start the engine, it may cause damage that would be far more expensive than the call-out fee.

How much does AdBlue cost?

The price of AdBlue/DEF can be a bit daunting at first, at around $32 per gallon at a dealer. You might find it cheaper online or at service stations, though. Also, keep in mind that it does improve fuel economy, so you may end up saving money.

Was this article helpful?
Please rate it
This article is rated 4.8 by 97 readers
Tips On How To Handle A Car On Fire
Your Guide To Crash Testing And Car Safety Ratings

Related Cars

To Top