This article will cover how to easily and accurately diagnose whether your diesel exhaust fluid (DEF) injector is in proper working order. An improperly working DEF injector will cause your truck to display a check engine light or "Reductive Fluid Quality" message on your dash. This can often cause your truck to go into limp mode and give you engine codes. These codes can vary depending on the truck but codes related to the DEF injector will always have to do with the NOx sensors or selective catalytic reduction (SCR) inefficiency. These codes or messages are often a sign 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 let the computer know to adjust the amount of DEF being added in order to mitigate the amount of NOx leaving the truck. If your injector is not adding enough DEF into the system, your truck's computer will recognize this as a lack of efficiency in the selective catalytic reduction (SCR) and this will be told by the NOx sensors. Luckily this problem is easily diagnosable since we simply need to make sure that the injector is flowing the proper metered amount of fluid.
Here's everything you'll need to perform the test:
The first thing you’ll want to do is remove the nuts or screws that connect your DEF injector to the exhaust. Once loose, you’ll want to pull the injector out while still leaving the electrical connection and fluid line connected to it. Now is a good time to visually inspect the injector. If there is any build-up or crystallization around the injector, cleaning or replacing the injector 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 a big help. 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 at all. If nothing comes out there could be a problem with the DEF pump, one of its connections, one of the injectors 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 your diagnostic unit is performing this test. Tank pressure should be around 70-75psi and pump percentage should be around 25%-30%, and 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 a lack of sufficient 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 flow rates for the DEF system since they are all trying to work to the same efficiency to combat NOx.
In that 2-minute span, your injector should put out somewhere between 40 and 55ml of DEF fluid to meet the required metered flow rate. A 2-minute test will give a large enough sample size to accurately tell that 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 crystalized build-up is to run the test over again a few times. This will hopefully break down and dissolve any crystalized DEF using the liquid DEF being sprayed out of the injector. However, this will often take multiple tries and a bit of 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 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 is more than 30g you will need to perform the diesel particulate filter service regeneration procedure. Along with that you will also need to run the "Reductant Fluid Quality Test" in order to prevent your check engine light from staying on. This will happen because the computer assumes that the inefficiency of the DEF is caused by its quality when reality it was the lack of DEF being sprayed causing the inefficiency. Doing these prosedues will save you from still displaying a check engine light and being stuck in limp mode even after remedying the problematic injector.
That's It, all you need to know on how to properly test a DEF injector. Hopefully, this will help you diagnose or rule out a bad or malfunctioning DEF injector on your diesel truck. While some precision is required this is a relatively easy test that can even be performed at home in your driveway.