What happens when you run your diesel out of exhaust fluid?


The last decade or so has been a real test for die-hard diesel vehicle owners, starting in 2007 with diesel particulate filters (DPF) and then again in 2010 with urea injection, also known as diesel exhaust fluid (DEF).

Remember the frustration, anger, and doubt diesel owners had when they were told that they not only have a more expensive exhaust system, but also a separate tank and injection system to keep track of that uses a mysterious magical fluid?

At the beginning of all this, I was a Master Technician for Mercedes-Benz. This was the comeback of the diesel. A way to comply with the new strict emission requirements. Inject a fluid before the catalyst and voilà - you have clean exhaust emissions.

DEF is made up of 32.5% urea and the rest is de-ionized water. It evaporates in the exhaust and when flowing through the catalyst, sets off a reaction that converts nitrogen oxide into nitrogen, water, and CO2 - all things we breathe in anyway. This reduces harmful NOX by up to 90%, and allows manufacturers to concentrate more on power and fuel economy thanks to the magic fluid taking care of everything post-combustion.


Sounds awesome, no? Well the general public and luxury car owners in particular saw it another way.  More money, more hassle. I mean, what would happen if you ran out of the fluid?

The penalty for running out completely is a harsh one.  Yes, there does come a time when you literally cannot start the engine because you are totally out. But you're given about a million warnings before the car gets its payback for you not caring for it properly.

For example, in the Range Rover Diesel. you get a 4.8-gallon tank which is good for about 6,300 miles before you are stranded. At 1,500 miles, the car begins to open a dialogue with you. A warning light shows you are getting low. These warnings get progressively more noticeable at 1,000 miles, again at 750, then 500, and a final warning at 100 miles. This last one is important. It tells you the vehicle will not restart until DEF is added.  If you get to this point, filling the tank is your only option unless you are prepared to never shut off the vehicle again.

Although most people won't ever actually have this happen, it shows how serious the EPA is on emissions. Well, for diesels at least.


One important question to ask would be, is this fair?

On a gasoline-powered vehicle, the car does not prevent you from starting it if the check engine light comes on. Even a flashing check engine light that could indicate damage to the catalytic converter does not stop you.  If the reason DEF-equipped cars won't start after you run dry is emissions, shouldn't gasoline cars be treated the same?

One thing is for sure: Knowing the repercussions of running out, DEF can also stand for Don't Ever Forget.  Oh, and don't let DEF leak out of its box. It will begin to look like a chemical Chia Pet.


Related: Watch these diesels destroy themselves