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DEF Header: What Are They and How Do They Work?

DEF Headers: What Are They and How Do They Work?

By: EquipmentShare

In 2010, the Environmental Protection Agency required all medium-duty and heavy-duty equipment manufactured to use DEF. You likely already use it in your machines, especially your newer ones.  Since it’s such an essential part of your machine, it’s helpful to know exactly what DEF is, how your machine uses it and why you should maintain your DEF system.

What is DEF?

DEF, or diesel exhaust fluid, is used in diesel-powered machinery to reduce the emissions the machine produces without sacrificing engine performance. The fluid itself consists of 32.5% urea and 67.5% deionized water. When injected into diesel exhaust, it converts harmful nitrogen oxides to water and nitrogen. It’s a necessary part of most machines’ everyday functions. Considering that the consequences of running out of DEF are severe, it should be a priority in the upkeep of your machine.

How Do Machines Use DEF?

The system that delivers DEF to your machine consists of a tank, a header and a catalyst. The DEF tank is where the fluid itself is stored. An L-shaped part called the DEF header is responsible for maintaining the integrity of the DEF and injecting it into the exhaust stream. These are also referred to as DEF sensors and DEF senders. Most of the header is inserted into the DEF tank. Finally, the diesel exhaust and the DEF are converted into their harmless byproducts in selective catalytic reduction, or SCR.

Multiple individual parts are required for a DEF header to do its job. Although every OEM creates different models, they all have two DEF lines – one for sucking DEF into the sensor and one to expel unnecessary fluid. Similarly, headers will have a system to regulate the temperature of the DEF in the tank. Most use coolant, but some have external heaters. DEF headers use sensors to monitor the DEF, including one for level, temperature and quality. Finally, most headers come with a suction filter to keep any debris in the tank from entering the sensor.

Dealing with Low DEF

Typically, you should refresh the DEF in your machine every 90 days. The EPA has more detailed instructions on when DEF should be replenished, but really, there’s never a bad time to refill. Sometimes, your machine might require more fluid before it’s time to refresh again. Your machine, thankfully, will help remind you when it is time.

The level sensor attached to the DEF gauge will activate a light on the dashboard to warn the operator DEF is at 10%. The light will typically start flashing as a more urgent reminder at 5%. By the time your DEF level is at 2.5%, the light will turn bright amber and the maximum speed of the machine will be heavily reduced. Not only is this inconvenient, but ignoring the DEF level will ultimately result in damaging the machine and reducing the engine’s performance.

Luckily, your DEF tank is easy to refill. Usually, the DEF sensor and DEF tank are located close to the diesel tank. They are color-coded with different caps, so it should be obvious which tank is for your fuel and which is for your DEF. The fluid itself can be found at automobile shops and hardware stores. Refilling the DEF in your tank is always a good idea, even if the interval has not arrived yet.

DEF tank cap (blue)

Common DEF Issues

Since the DEF gauge is made up of several smaller parts, it is not unusual to come across maintenance issues. Most issues can be resolved easily, but should never go unchecked. Neglecting to notice and take care of these issues could ultimately lead to a breakdown – costing you time and money. Here are a few common problems that can cause your DEF sensor to fail.

Coolant Leaks

Diesel exhaust fluid needs to stay at a consistently cool temperature. DEF headers use coolant to do this. It is very easy for the coolant tubes to spring leaks. Not only are you losing coolant, but contaminants like dirt can sneak into those small spaces, which is never good. If you shine a flashlight through your coolant tubes and they are murky, you likely have a leak on your hands.

Dirt Contamination

It is of the utmost importance that your DEF is pure. Although the DEF header is equipped with filters to help prevent debris from passing, it can still occasionally slip through, which will cause a failure. The best way to inspect for contamination is to check the fluid itself. DEF is clear: if your DEF is not, something is definitely wrong.


In general, DEF can be a very fickle fluid. If there is the presence of minerals in your DEF, such as calcium, it’s possible crystals will gradually form as you use your machine. Inevitably, that would lead to a DEF header failure. If you notice any suspicious formations in your tank or header, it might be time to switch your source of DEF (and give your DEF header a good cleaning).

Urea Concentration

About one-third of the DEF solution consists of urea. If your solution is too concentrated and too much urea is present, the heat from the machine can turn it into ammonia. The ammonia would go on to burn and deteriorate not only the DEF header, but the entire machine. To prevent the creation of ammonia, you should test the concentration of urea in your DEF. It should be somewhere between 31% to 33%, but the ideal spot is 32.5%.

Failing Sensors

A DEF header requires all of its sensors in order to keep both the system and the fluid in good condition. If those sensors are broken, the DEF header simply cannot operate. You will know if the reason your DEF header isn’t working is a sensor issue because the machine will throw an error code depending on which sensor is out of commission. Luckily, replacement sensors are available online.

Trust the Experts

Understanding how DEF works is key to a healthy machine with a long life. A lot can go wrong with your DEF header, but every problem has a solution. When it comes time to work on your machine, you’ll find the parts you need in EquipmentShare's Online Parts Catalog. With OEM original parts and fast shipping, you can get exactly what you need to get up and running again.