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What Happens When you Run out of Diesel Exhaust Fluid

February 19, 2024 | Victor Lukasso

Do you care to know what will happen if your Diesel powered vehicle runs out of DEF? Continue reading to find out.

Diesel engines use diesel exhaust fluid (DEF), a urea-based solution, to keep the car running and decontaminate the harmful substances from the exhaust.

It is used in selective catalytic reduction (SCR) systems to reduce nitrogen oxide (NOx) emissions in diesel-powered vehicles.

When injected into the exhaust, the fluid releases a chemical reaction that converts harmful pollutants into clean air.


It is a key component in helping these vehicles meet emissions standards and is stored in a separate tank.

So, it’s required that any diesel-powered vehicle running above 74 horsepower should have the SCR systems and also have DEF.

When a vehicle runs out of DEF, the check engine light will turn on; you’ll experience reduced engine performance and damage to components such as the DEF Header.

In this article, we’ll further elaborate on what Happens When you Run out of Diesel Exhaust Fluid (DEF)

What Happens When You Run Out of Diesel Exhaust Fluid

It’s important to note that the consequences of running out of DEF can vary depending on the specific vehicle and its SCR system.

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Some systems may be more tolerant of a lack of DEF than others, while others may be more sensitive and experience more severe consequences.

Here are some potential consequences of running out of DEF:

1. The Check Engine light will turn on

The first sign that comes up when the DEF tank empties is a warning light on the dashboard.

If you use advanced monitoring systems, you might also receive emails or text alerts with details of the car that needs attention.

2. Damaged DEF Sensor

While the problem can often be fixed by simply refilling the tank with DEF and starting your engine, the DEF Sensor, which keeps track of the fluid temperature, level, and quality, can be fluid.

This might not occur at first, but due to frequent drying up of the diesel exhaust fluid and this could cost about $1,200 to install a replacement.

So, it’s advisable always to keep an eye on your DEF and ensure it never runs dry.

3. Reduced Performance

If a vehicle’s SCR system is not functioning properly due to a lack of DEF, it may experience reduced performance. This can manifest in various ways, such as decreased power and torque, slower acceleration, or reduced fuel efficiency.

4. Increased emissions

A VEHICLE’S EMISSIONS MAY INCREASE without DEF to help convert NOx emissions into harmless nitrogen and water vapor.

This can not only harm the environment but also result in the vehicle failing emissions tests and potentially facing fines or penalties.

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5. Clogged Exhaust System

If you keep running out of diesel exhaust fluid, soot, and debris will eventually clog the exhaust system making it less efficient.

6. Damaged SCR system

If a vehicle continues to be driven without DEF for an extended period, it can potentially cause damage to the SCR system.

Replacing a damaged SCR system is usually between $2,500 to $5,000.

7. Vehicle Break down

In some cases, if a vehicle runs out of DEF, it may simply shut down and be unable to be driven until more DEF is added.

In some cases, this could also be accompanied by a series of repairs due to vehicle damage.

That’s all.

To avoid running out of DEF and experiencing these negative effects, drivers must pay attention to their DEF levels and refill the tank when necessary.

DEF Tips

  1. Vehicle owners can take advantage of the DEF gauge or warning light that indicates when the DEF level is running low and refill the DEF tank before it reaches empty.
  2. DEF should be available at most diesel fuel stations and can be purchased at auto parts stores or online.
  3. Always have a container of DEF in your car in emergencies.
  4. It is generally recommended to use high-quality DEF that meets the ISO 22241 standard, as lower-quality DEF may not work as effectively and could cause problems with the SCR system.
  5. DEF, like other fluids, can expire over time, especially when stored for a long period or exposed to high temperatures. This would reduce the effectiveness of the fluid, and it can even cause problems for the SCR system.
  6. It’s a good idea to check the expiration date on the DEF before using it and to store it in a cool, dry place to extend its lifespan.
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Is it bad to run out of DEF?

It’s not advisable to let your vehicle run out of DEF. This could cause your engine’s reduced performance and incur heavy charges from repair if not addressed immediately, plus your vehicle will fail emissions tests.

What happens when diesel exhaust fluid is low?

If your Diesel exhaust fluid is low, your engine performance will be degraded until you add DEF to the empty tank. It could result in several damages if not fixed on time.

What happens if you run a diesel without DEF?

If you run a diesel without DEF, the car will suffer from limp mode or not respond, depending on the vehicle and the SCR system.

Can I use water instead of DEF?

No, DEF is a urea and water mixture that uses selective catalytic reduction (SCR) systems to reduce nitrogen oxide (NOx) emissions in diesel-powered vehicles.

Water or urine can’t serve this purpose in replacement of a DEF.

How many miles can you go on a DEF fluid?

The distance a vehicle can travel on a diesel exhaust fluid (DEF) tank will vary depending on several factors, including the size of the DEF tank, the vehicle’s fuel efficiency, and the amount of DEF used by the selective catalytic reduction (SCR) system.

Generally, depending on these variables, most vehicles can travel between 3,000 and 5,000 miles on a single DEF tank. However, it’s important to note that this is a rough estimate, and actual mileage may vary.

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Meet Victor Lukasso, the owner of V. Auto Basics. Through this blog, Victor Provides Insights on the latest tips, maintenance, repair, and techniques in the automotive world.