How does DEF work in SCR systems?

Here's a simple breakdown of what DEF does in SCR systems. First, DEF is injected into the hot exhaust stream, where it decomposes into ammonia. The ammonia reacts with nitrogen oxides (NOx) in the exhaust gas inside the SCR catalyst, converting harmful NOx into harmless nitrogen gas (N2) and water vapor. The vehicle's electronic control unit adjusts DEF injections based on exhaust conditions, engine load and NOx levels to maintain optimal emissions reductions.

Where do I refill the DEF tank on my truck?

The DEF tank is usually located near the diesel fuel tank and is identified by a blue cap. You can refill DEF at truck stops with DEF pumps, gas stations, auto parts stores or using portable jugs. Always ensure the nozzle and cap are clean before refilling to prevent contamination.

Can I use automotive urea instead of DEF?

No, you cannot use automotive urea or any other urea solution instead of certified DEF. DEF must meet strict ISO 22241 standards for purity and composition. While DEF contains urea, it requires a precise 32.5%concentration and high-purity deionized water. Using noncertified urea solutions can contaminate SCR systems.

How long does DEF last in storage?

When stored properly, DEF has an expected shelf life of 12 to 18 months from the manufacturing date. While it's a stable chemical solution, the urea concentrate within it can decompose and degrade over time, especially if not stored under optimal conditions.

Can you run a diesel without DEF?

No. Modern diesel vehicles equipped with SCR systems (most diesels manufactured after 2010) cannot operate normally without DEF.

Can you make your own DEF fluid?

No, it is not recommended for most people and businesses to make their own DEF, even though it is technically possible. Some large fleet operators or specialized industrial facilities might be able to produce their own DEF under strict quality control. However, for individual vehicle owners or small businesses, purchasing commercially produced, API-certified DEF is the safest and most reliable option. The savings from DIY are rarely worth the risk of causing costly damage to a vehicle's emissions system.

Can I run water instead of DEF?

No, you absolutely cannot run water instead of DEF in a modern diesel vehicle. While DEF is mostly water, it is the precise 32.5%urea content that allows the SCR system to function correctly.

BlueDEF is like AdBlue — it is another brand of DEF. BlueDEF is marketed as being compliant with ISO 22241-1 specifications for DEF purity and composition. It is also often American Petroleum Institute-certified, indicating it meets industry standards for quality and performance.

Fleet Optimization

What is DEF? How diesel exhaust fluid works and why it's essential for fleets

Diesel exhaust fluid (DEF) is a nontoxic, colorless liquid consisting of 32.5% high-purity urea and 67.5% deionized water. It is primarily used in diesel vehicles to help reduce harmful emissions.

By Kevin Buchaly

Software Project Specialist at Geotab

Jul 17, 2025

Key Insights

DEF is a solution of urea and deionized water that reduces nitrogen oxide (NOx) emissions from diesel engines by up to 90%.
Diesel vehicles use selective catalytic reduction (SCR) systems to inject DEF into the exhaust system to convert harmful NOx into nitrogen gas and water vapor.
Running out of DEF triggers "limp mode," which reduces vehicle speed and performance until the tank is refilled.
Store DEF in a cool, dry place away from sunlight to maintain fluid quality and prolong its shelf life.

Fleet management comes with endless responsibilities. From vehicle maintenance to insurance costs, emissions are not always top of mind. But with the ability to reduce nitrogen dioxide emissions by up to 90%, DEF is an important consideration for diesel fleets.

Understanding what DEF is, how it works and how it impacts the environment and your fleet helps you maintain efficient operations. Read on to learn how DEF systems can support regulatory compliance, reduce environmental impact and keep fleets running smoothly.

What is diesel exhaust fluid (DEF)?

DEF is a colorless, nontoxic liquid composed of 32.5% high-purity urea and 67.5% deionized water. It is used in diesel vehicles equipped with selective catalytic reduction (SCR) systems. When added during refueling, it can help reduce harmful nitrogen oxides (NOx) emissions.

DEF vs. diesel

Diesel fuel and DEF for diesel trucks serve completely different purposes and should not be mixed. Understanding the difference between the two can help prevent costly mistakes and ensure smooth vehicle operations.

Feature	Diesel fuel	DEF
Purpose	Powers the engine through combustion	Reduces NOx emissions in the exhaust system
Consumption rate	Primary fuel consumed during vehicle operation	Typically 2% to 3%of diesel fuel consumption
Handling	Flammable and requires standard fuel handling procedures	Nontoxic but must be kept clean to prevent contamination
Composition	Petroleum-based hydrocarbon fuel	32.5% urea and 67.5% deionized water
Temperature sensitivity	Can gel in extremely cold temperatures	Freezes at 12°F (-11°C) but thaws automatically as the vehicle warms
Refill frequency	Based on mileage and fuel efficiency	Approximately every 5,000 to 7,000 miles for light-duty vehicles

What is diesel exhaust fluid used for?

DEF is primarily used to chemically convert NOx into harmless nitrogen gas and water vapor. This process is crucial for meeting stringent environmental regulations and emissions compliance standards, such as EPA 2010 and Euro VI, which aim to significantly lower pollutants from diesel engines.

Beyond environmental benefits, using DEF properly contributes to optimal fuel efficiency and vehicle performance. The SCR system, enabled by DEF, allows the engine to be tuned for greater power output without being constrained by NOx emission limits. This results in better fuel economy and sustained engine performance over the vehicle's lifespan, as the engine can operate in its most efficient range.

For fleets, proper DEF management means maintaining regulatory compliance and keeping vehicles operational. Vehicles that run out of DEF or experience SCR system failure cause unplanned downtime, impacting revenue and customer satisfaction. That is why using fleet optimization tools to monitor refill and maintenance schedules is so useful.

Keep your fleet running smoothly and compliant with smart DEF management and maintenance scheduling.

What is DEF made of?

Diesel exhaust fluid is composed of 32.5% urea and 67.5% deionized water. Unlike N2, which is safe and fairly inert, NOx can pose health and environmental risks when inhaled. Reducing NOx in the atmosphere, particularly in areas where we live and work, is why diesel engines use DEF.

Gasoline engines run cooler, so they typically produce less NOx than their diesel counterparts. As a result, there is less pollution from NOx compounds, and therefore, gasoline engines do not need to use SCR to reduce NOx in the exhaust.

Illustration showing the two components of diesel exhaust fluid: 32.5% urea and 67.5% deionized water

How does DEF work?

Here is a step-by-step breakdown of how DEF works:

In the SCR system, the hot exhaust gases leave the engine.
A precise amount of DEF is injected into the exhaust stream.
The heat from the exhaust causes the DEF to decompose into ammonia (NH3) and carbon dioxide (CO2).
The ammonia travels into the SCR catalytic converter.
Inside the SCR catalyst, the ammonia reacts chemically with the NOx present in the exhaust gas.
This reaction converts the hazardous NOx into harmless nitrogen gas (N2) and water vapor (H2O).
Both are safely released into the atmosphere.

What happens if you run out of DEF?

Before a vehicle runs out of DEF, there will be warnings and the vehicle will go into what is called “limp mode” until the tank is refilled. Limp mode reduces speed and performance.

To ensure optimal performance and compliance with emission standards, modern diesel vehicles are equipped with onboard DEF monitoring systems. These systems continuously track the DEF level, purity and injection rate. If the fluid is low or there is a system malfunction, dashboard lights or messages alert the driver and may trigger limp mode to prevent excessive emissions.

Some advanced fleet management systems integrate AI fleet management to predict when vehicles will need DEF based on usage patterns and consumption rates. This allows fleets to schedule refills proactively, before warnings even appear.

How often do you need to refill DEF fluid?

Generally, light-duty diesel vehicles may need a DEF refill every 5,000 to 7,000 miles, or roughly every three months (often coinciding with oil changes). For vehicles with a 5-gallon DEF tank, this could mean refilling every third or fourth time you fill up your diesel tank.

DEF refill frequency varies significantly depending on several factors, including:

Vehicle type and size: Heavy-duty trucks and equipment generally consume more DEF than light-duty diesel vehicles due to their larger engines and more demanding operating conditions.
Engine size and load: Bigger engines working harder (e.g., towing heavy loads, driving uphill, experiencing frequent stop-and-go traffic) will produce more NOx and thus require more DEF.
Driving conditions and style: Aggressive driving with rapid acceleration, harsh braking and high speeds increases DEF consumption. Smoother, steadier driving can help conserve DEF.
DEF tank size: Obviously, larger DEF tanks allow for longer intervals between refills.

Diagram showing how diesel exhaust fluid (DEF) works in an SCR system, from injection to NOx conversion, with fleet monitoring and refill considerations.

How to store DEF

Exposure to high temperatures, particularly above 90°F (32°C), can significantly accelerate DEF’s degradation and reduce shelf life to as little as six months. For best results, store DEF:

In a cool, dry and well-ventilated place
Out of direct sunlight
At temperatures between 12°F (-11°C) and 86°F (30°C)
In plastic to avoid corrosion

While not classified as hazardous, DEF should be handled with care to prevent contamination and spills. Use dedicated, clean equipment for dispensing, and wear gloves to protect sensitive skin, as prolonged contact can cause mild irritation.

Keep your fleet running clean with Geotab

Beyond emissions reduction, diesel exhaust fluid is an important component of fleet optimization, keeping diesel vehicles running smoothly and efficiently. Managing DEF as part of a comprehensive fleet maintenance strategy helps you reduce downtime, maintain compliance and control operating costs.

Geotab's fleet management solutions give fleet managers the tools they need to monitor vehicle health, track maintenance schedules and optimize operations. Telematics data can alert managers to low DEF levels before they become critical, and integration with maintenance systems ensures DEF refills are always scheduled proactively.

Ready to optimize your fleet's DEF management and overall operations? Learn more about fleet optimization with Geotab’s range of fleet management solutions.