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What Does "Poor DEF Quality" Mean?

The dreaded warning messages, they always lead to the same thing, lost revenue and unexpected expenditures. So, you are driving, and you receive a message on the dash reading "Poor DEF Quality." What does this mean? What should you do about it, or not do? By the end of this blog post, our hope is that you will have a clear understanding as to what this message is suggesting and how you can go about getting yourself back up and cranking out the miles as efficiently as possible  

"Poor DEF Quality"...may not always mean your DEF quality is bad  

To start, "Poor DEF Quality" is highly suggestive and worded in a way that can be misleading. I believe the best way to word this message would be "High NOx Emissions Detected" or "Poor SCR Efficiency" (some mfg use this terminology). Either of these would be a more accurate representation of what is wrong and better encompasses its potential causes. In short, this message means that the NOx emissions measured at the outlet NOx sensor are higher than the computer expects for a given situation.    

What can cause this message to come up?   

The potential causes are a more extensive list than the dashboard message suggests. While it is common that the root cause is poor DEF quality, it is not a guaranteed cause by any means.   

Potential causes beyond bad DEF fluid include:   

  • Bad NOx sensors  
  • Contaminated SCR box 
  • DEF dosing issues 
  • Potential EGR issue 

Diagnosing through the list can be tough if you have nothing else to work with. But if you have any recent trouble codes, they could give you some direction as to the best place to start your diagnosis.   

If this message arrived shortly after a DEF fluid refill, the easiest thing you can do is to test your DEF fluid. Is it good? If it is good, what should you do next? The next thing to do is a thorough visual inspection. Look over the entire exhaust and DEF systems, including the EGR all the way through to the SCR. Any visible leaks, excessive corrosion, or unusual wear typically suggests a problem area.

If your truck has not received all its factory recommended maintenance, a thorough tune-up would make sense. A new DEF filter, EGR tune-up, and a fresh set of NOx sensors can go a long way. It can eliminate the potential of skewed sensor readings or poor EGR performance, and it can assure you that you'll have the correct information feeding the computer for future diagnosis if the problem continues.

After a tune-up, a recent code history could help if you had some recent issues, but no items replaced, it may be the root of your headaches. For example, if you had a DEF pump code recently, it might make sense to diagnose the DEF injection system. Beyond that, you will want to start digging into the individual components and testing each one.    

While there is no way to eliminate all the potential issues, maintenance does wonder in the prevention of unexpected problems. During the diagnosis process, OTR Diagnostics can be used to put the truck into a forced DPF regeneration, which is often a part of diagnosing aftertreatment issues. On top of being a way to test repairs and confirm their success, OTR Diagnostics is an affordable alternative for you to take control of the system and remove derates associated to this problem without a costly visit to the dealership.     

Until next time! 👋
Stay safe, stay healthy & be blessed.

Looking to learn more? Check out our other blog posts.

OTR Diagnostics allows you to read and reset fault codes, view live data, and run advanced diagnostic functions, including forced DPF regen with your mobile phone or tablet. 

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