Diesel Exhaust Fluid (DEF) plays an important role in moderating truck emissions and meeting EPA guidelines. As with any type of vehicle fluid, regular monitoring of DEF levels is an important part of ongoing maintenance. While checking diesel exhaust fluid levels isn’t a difficult job, there are factors that can complicate the issue. Here’s a quick overview of everything you need to know.
Table of Contents
- DEF Basics
- How to Check DEF Fluid Level
- Topping Off
- How Long Does Diesel Fluid Last?
- How Much DEF Do I Need?
- DEF Hazards
- Frequently Asked Questions
DEF Basics
Diesel exhaust fluid is required for a process called selective catalytic reduction (SCR). SCR works by diverting exhaust gases through a filter and then into a catalyst. There, the gases mix with DEF, which turns hazardous NOx emissions into a harmless mixture of nitrogen and water.
Diesel exhaust fluid has to be replaced on a regular basis. Replacement intervals are tied to the fuel efficiency of your engine. Less efficient engines produce more exhaust gases, which require more DEF to treat.
Typically, dose rates for a well-maintained heavy- or medium-duty truck engine will be between 2-3%. This means that a truck getting six mpg will require between three to five gallons of DEF for every 1,000 miles it travels.
How to Check DEF Fluid Level
All truck engines meeting 2010 or later EPA guidelines are required to have a dashboard warning system that alerts you when DEF levels start running low. These systems feature an amber warning light that goes off when levels drop below 10%. At 5%, the light starts flashing, and when levels drop below 2.5%, it turns solid. When DEF runs out entirely, the light turns red, and the truck is electronically limited from going more than five mph.
DEF levels can also be checked manually. The storage tank can typically be found next to the diesel fuel saddle tank on a truck, or in the spare tire area of a passenger vehicle.
Topping Off
Replacing DEF is fast and easy — even if a level check doesn’t indicate that you’re running low, it’s always best to fill up before a long haul and avoid the risk of a problem. DEF can be found in all truck stops and auto parts stores. It is a non-toxic liquid that can be handled without any special protective clothing or tools.
To add more DEF, simply locate the fill port — it is usually found under the hood, next to the fuel port or in the trunk — and top it off manually. Fuel and DEF ports are differentiated in size to reduce the risk of accidentally putting one into the other.
How Long Does Diesel Fluid Last?
Diesel exhaust fluid normally has a shelf life of about two years. However, exposure to sunlight or sustained high temperatures can compromise it. Any DEF purchased at a truck stop or auto parts store will have a clearly labeled expiry date — using DEF past this date can cause performance issues and potential maintenance problems.
‘Check DEF’ Message
When the dreaded “Check DEF” message appears on the instrument panel of your Sprinter van, you need not panic. The AdBlue DEF system has to do with your Sprinter’s emissions and it does not affect the performance of the engine. The “Check DEF” message means that either there is a level sensor fault inside the DEF tank or the DEF computer has detected a fault with one of the injectors or tank heaters. However, if you ignore the DEF issue, you will get a warning that there will be 20 engine starts left and it begins a countdown each time the engine is started.
What is AdBlue?
AdBlue is a harmless substance. It is not a fuel, nor a fuel additive but a high-purity urea solution that must be used in a dedicated tank on your Sprinter van, that should be refilled before running low. AdBlue is non-flammable and non-explosive. It is classified as a transportable fluid with minimal risk. The purpose of Adblue is to reduce the amount of harmful Nitrogen Oxide (NOx) found in your Sprinter’s emissions by turning it into nitrogen, carbon dioxide and water. Mercedes-Benz has managed to significantly reduce exhaust gas emissions while at the same time maintaining the performance of diesel engines in terms of power and torque output. However, the Sprinter DEF system is complex and expensive.
The Sprinter Van DEF System
Mercedes designed a system by which the fumes pass through an Exhaust Gas Recirculation (EGR) valve before they enter the exhaust system. The exhaust system is made up of a CDI (control unit) which monitors and activates the sensors and valves in the exhaust system. In the engine compartment of your Sprinter Van, there is a DEF tank, a DEF temp sensor, a DEF level sensor and a Selective Catalytic Reduction (SCR) control unit with actuators and sensors. The actual exhaust system has three major components: the OXI-Cat and DPF section and two SCR Cat sections. The OXI-Cat section has an oxygen sensor an exhaust temperature sensor and a backpressure sensor. There is an N0x sensor and a dosing valve between the OXI-Cat section and the first SCR Cat section. In the first SCR Cat section, there is another temperature sensor, and the last SCR section has one more N0x sensor. Once the exhausted fumes and particles have passed through this area, they are passed through a particulate filter.
How We Can Help
As you can see, the system is quite complex and if any of these parts fail, you may not be able to use your Sprinter van. What’s more, if you decide to take your Sprinter to a local dealership, you can expect high repair bills, both for parts and labour. Here at Norlang Auto, our factory trained technician have the experience, knowledge and proper equipment to accurately diagnose and test this complex system. Best of all, our repair bills will only be a fraction of the cost of what you’d normally pay at a dealership. Our technicians are also able to reset the warning countdown before you are prevented from starting up the engine. An important reminder for all owners of vehicles with the AdBlue DEF system is to avoid allowing the liquid to reach low levels since it can cause the pump and heater to fail and prevent you from starting your vehicle. For more information about this vital system, please visit or call us at Norlang Auto.