How to Perform a DEF Injector Test

An improperly functioning DEF injector will cause your truck to display a check engine light or "Reductive Fluid Quality" message on your dashboard. This can often cause your truck to enter limp mode and display engine codes. These codes can vary by truck, but DEF injector codes will always relate to NOx sensors or selective catalytic reduction (SCR) inefficiency. These codes or messages often indicate that the DEF injector is not functioning properly.

The injectors in the DEF system are designed to inject a metered volume of DEF fluid into the catalyst. The DEF reacts with NOx in the catalyst, turning it into nitrogen, H2O, and CO2. The NOx sensors upstream and downstream of the catalyst measure the amount of NOx entering and exiting the catalyst. The sensors then signal the computer to adjust the amount of DEF added to mitigate NOx emissions from the truck. If your injector is not adding enough DEF to the system, your truck's computer will recognize a lack of efficiency in selective catalytic reduction (SCR), which the NOx sensors detect. Luckily, this problem is easily diagnosable, since we simply need to ensure that the injector is delivering the proper metered amount of fluid.

Here's everything you'll need to perform the test: 

• 10mm Wrench/socket
• Diagnostics scan tool with test functionality. 
• Graduated Cylinder or a way of measuring ml of liquid. 
• Funnel (Preferred)

The first thing you’ll want to do is remove the nuts or screws that connect your DEF injector to the exhaust. Once it's loose, you’ll want to pull the injector out while keeping the electrical connection and fluid line attached. Now is a good time to visually inspect the injector. If there is any buildup or crystallization around the injector, cleaning or replacing it will be necessary. We will go over cleaning the injector at the very end of this article.

With the injector removed from its port, hang it into/over the container you plan to measure the total amount of DEF collected in. This is where using a funnel or having someone else under your truck to hold the container while you start the test is especially helpful. You will need to catch every drop being released from the injector for your test to be accurate.

We can now plug our scan tool into the truck and start the "Reductant Injection Quantity Test". On our scan tool, it was located under powertrain, service functions, SCR system, SCR dosing measurement test. This test will run the DEF injection system for 2 minutes straight. This will help us in multiple ways. Firstly, it will let us see whether any fluid is coming out of the injector. If nothing comes out, there could be a problem with the DEF pump, one of its connections, one of the injector's connections, the injector itself could be broken, or it could be a broken/damaged DEF line. In this case, it's also a good idea to keep an eye on your reduct (DEF) tank pressure and pump % while the diagnostic unit performs this test. Tank pressure should be around 70-75 psi, and pump percentage around 25%-30%. This can help you tell if your pump is in proper working order.

While you're performing the diagnostic test for the injector system, you’ll also want to take a close look at the spray pattern and make sure that the DEF is coming out in 3 individual streams, each with a uniform spray pattern. A clogged or partially clogged injector hole will cause inconsistent spray patterns, leading to insufficient flow. Once we have performed the test and are satisfied that the DEF is being sprayed out of the injector in a constant, uniform pattern. We want to ensure that our flow is being metered precisely. This is where the fluid you've collected comes in. All SCR dosing measurement tests or DEF injector tests should run for approximately 2 minutes as a control. All trucks use the same DEF flow rates because they are all aiming for the same efficiency to combat NOx.

In that 2-minute span, your injector should deliver between 40 and 55ml of DEF fluid to meet the required metered flow rate. A 2-minute test will provide a large enough sample size to accurately determine whether the injector is consistently metering that flow rate over a long period. If the measured amount of DEF collected is within range, then your injector is in proper working order and is not the cause of your check engine light or codes. On the other hand, if you measured less than 40ml, you may need to clean or replace your injector. However, if the volume is greater than 55ml, you will need to replace your injector, no questions asked. 

A simple way to unclog an injector with crystallized buildup is to run the test over again a few times. This will hopefully break down and dissolve any crystallized DEF, with the liquid DEF being sprayed out of the injector. However, this will often take multiple tries and some time. The best way to dissolve built-up DEF crystals is by using a steam cleaner. The hot steam will do a great job at penetrating, breaking up, and dissolving the crystals.

After you have identified that the injector is not working properly and have cleaned or replaced it. You will want to check the diesel particulate filter soot mass on your scan tool before starting up and driving your truck. If the soot mass exceeds 30g, you will need to perform the diesel particulate filter service regeneration procedure. Along with that, you will need to run the "Reductant Fluid Quality Test" to prevent your check engine light from staying on. This will happen because the computer assumes that the DEF's inefficiency is due to its quality, when in reality, it was the lack of DEF being sprayed that caused the inefficiency. Doing these procedures will save you from still displaying a check engine light and being stuck in limp mode even after remedying the problematic injector.

That's all you need to know to properly test a DEF injector. While some precision is required, this is a relatively easy test that can even be performed in your driveway at home.