How to manage diesel exhaust fluid (DEF) and reduce downtime

Diesel exhaust fluid is easy to overlook until a low-fluid warning or system fault starts affecting vehicle performance. For diesel vehicles equipped with selective catalytic reduction (SCR), running out of DEF or ignoring certain system problems can eventually trigger a reduced-performance state commonly known as “limp mode.”

Understanding how DEF works and incorporating it into routine maintenance can help fleets support asset uptime and manage avoidable delays.

What is diesel exhaust fluid?

Diesel exhaust fluid (DEF) is made of 32.5% high-purity urea and 67.5% deionized water. It is used in diesel vehicles equipped with selective catalytic reduction (SCR) systems to help reduce nitrogen oxide (NOx) emissions.

It became common in U.S. diesel vehicles beginning around 2010, when stricter EPA NOx emissions standards led manufacturers to widely adopt SCR systems that use DEF.

The diesel exhaust fluid is stored in a separate tank and is not mixed with diesel fuel. Depending on the vehicle and emissions system, DEF may be used in light-, medium- and heavy-duty diesel vehicles as well as some diesel-powered equipment.

What does diesel exhaust fluid do?

The DEF purpose for fleets does not fuel or lubricate the engine. Its primary job is emissions treatment – it supports the exhaust aftertreatment system that helps diesel vehicles meet emissions requirements.

Watch this video to learn how telematics makes managing vehicle health and emission simpler.

How does diesel exhaust fluid work?

A DEF system works using SCR technology to treat exhaust after it leaves the engine.

The basic process works like this:

• Hot exhaust gases leave the diesel engine.
• A diesel exhaust fluid pump moves DEF from the tank to the dosing system, which injects the appropriate amount into the exhaust stream.
• Heat causes the urea in DEF to break down and produce ammonia.
• Inside the SCR catalyst, ammonia reacts with NOx in the exhaust.
• The reaction converts NOx primarily into nitrogen and water vapor before the exhaust is released.

Because DEF works inside the exhaust system, it should never be added to the diesel fuel tank.

What happens if you run out of DEF?

Modern diesel vehicles generally provide low DEF warnings before the DEF tank is empty. If the driver continues operating the vehicle without stopping to refill diesel exhaust fluid, the vehicle may progressively restrict performance.

Depending on the manufacturer and vehicle, restrictions can include reduced engine power, limited speed or difficulty restarting the vehicle after shutdown.

The vehicle may begin to limit engine power or speed, a process known as derating. This reduced-performance state is commonly called a derate, “limp mode” or sometimes “limp-home mode.” The vehicle has enough power to return home or to a service station, but will need work once it's there. The exact sequence is controlled by the vehicle manufacturer, so there is no single universal version of DEF limp mode.

Can you run a diesel without DEF?

If the vehicle was designed to use DEF, usually not. Its emissions controls are designed to warn the driver and eventually restrict operation when qualifying DEF or SCR problems are not corrected.

What DEF problems can trigger warnings or reduced performance?

Running out of fluid is only one potential DEF problem that could lead to DEF system failure. Contaminated DEF, sensor problems, pressure loss and SCR system failures can also generate diagnostic trouble codes (DTCs).

Some generic codes associated with DEF and aftertreatment problems include:

These are examples of industry-standard generic codes. Manufacturer-specific codes and the sequence that leads to a derate or limp mode can vary by vehicle.

A DTC also does not necessarily mean the vehicle is already operating in full limp mode. Some faults may be detected earlier in the sequence while others may appear closer to or during a derate.

How often should you refill diesel exhaust fluid?

There is no universal DEF refill interval. How often you put DEF in a diesel truck depends on the vehicle, engine load, duty cycle, driving conditions and DEF tank capacity.

DEF use can vary based on:

• Vehicle and engine size
• Towing or payload demands
• Route conditions and duty cycle
• DEF tank capacity
• Overall diesel fuel consumption

How often do you put DEF in a diesel truck?

DEF consumption is typically about 2% to 3% of diesel fuel consumption,¹ although actual use varies by vehicle and operating conditions. That means there is no single refill interval that applies to every diesel truck.

For fleets, incorporating DEF checks into routine inspections and preventive maintenance can also make replenishment more predictable.

How should you store and handle DEF for trucks?

DEF must remain clean and properly formulated to work correctly in an SCR system.

When storing or dispensing diesel exhaust fluid:

• Use clean, dedicated containers and equipment.
• Keep DEF away from diesel fuel, tap water and other contaminants.
• Protect stored fluid from excessive heat and direct sunlight.
• Follow manufacturer and supplier recommendations for storage temperatures and shelf life.

Industry focus: DEF for construction equipment

DEF isn't limited to on-road diesel trucks. Many newer diesel-powered construction assets use SCR systems and require DEF used for off-road equipment.

The operational stakes can be different on a job site. A DEF or SCR system problem that reduces equipment performance can sideline a critical asset and disrupt scheduled work. Construction fleets can benefit from including DEF checks in routine asset inspections to identify emissions-system issues early.

GPS tracking and supported engine data can also help managers monitor equipment utilization, coordinate maintenance and keep track of powered assets across job sites.

 Keep your construction fleet moving and your jobs on track. See how intelligent tracking can help boost uptime and profitability.

DEF freezes at about 12°F (-11°C), but properly formulated fluid can generally be used after it thaws. Dried fluid may also leave white crystalline deposits around tank caps, fittings or dosing components. Significant diesel exhaust fluid crystallization within the system may interfere with normal operation and require maintenance.

Can you put water in the DEF tank instead?

No. DEF systems require a specific concentration of diesel exhaust fluid ingredients: high-purity urea and deionized water. Diluting it with additional water or contaminating it with other fluids can trigger DEF quality faults and interfere with SCR performance.

Verizon Connect integrates with expanded maintenance programs through its robust list of partnerships. See the connections here.

How can fleets reduce the risk of DEF-related downtime?

Telematics cannot replace the DEF gauge or guarantee that a driver will refill diesel exhaust fluid. DEF management works best as part of a broader preventive maintenance management program that helps fleets stay on top of inspections, scheduled service and vehicle faults.

Verizon Connect can support that process in several ways:

• Streamline routine inspections and DEF checks. Fluid levels, warning indicators, visible leaks and crystallization can be incorporated into daily vehicle inspection reports (DVIRs), giving drivers a consistent process for documenting vehicle condition and helping maintenance teams identify reported issues that need attention.
• Create service plans based on vehicle use. Fleets can set up service plans for vehicles and assets that include routine DEF maintenance and other recurring service needs. Maintenance is documented and can be scheduled using mileage, engine hours, time or other supported intervals, with automatic reminders sent to the appropriate personnel when service is due.
• Track maintenance and emissions issues. Reveal reports provide a snapshot of scheduled service, maintenance costs and vehicle issues that need attention. The Emissions System Report also identifies emissions-related faults and DTCs for supported light- and medium-duty vehicles, helping teams coordinate repairs and make better use of planned shop time.
• Respond quickly when engine or emissions faults appear. Reveal Engine Connect can bring supported engine data, including diagnostic trouble codes (DTCs), into Reveal. Fleets can set alerts to notify designated personnel when new DTCs are received and flag important issues as high priority, helping maintenance teams respond faster when problems arise. OEM integrations can also bring supported vehicle data directly into Reveal without additional hardware installation.

Diesel exhaust fluid is only one maintenance requirement competing for attention across a diesel fleet. A structured maintenance program can help teams manage DEF alongside oil changes, inspections, emissions-system issues and other service needs that can affect vehicle availability.

Want to simplify vehicle maintenance so you’re not caught off guard with a low DEF fluid warning or breakdown? Read our master guide to maintenance and vehicle health to learn how to boost uptime and shrink maintenance costs.

Sources

¹ Don't Forget to Factor DEF into Fuel Costs, Heavy Duty Trucking