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My 2015 Sierra Denali 12,000 miles. DIC came up indicating I was less than 100 miles to restricted speed, fill DEF. Took it to dealer for refill, guy filled it, wouldn't clear message, checked codes, showed something else is wrong, tech determined truck needs new catalytic converter. Waiting on part, 12 days from Canada, and GM Engineering is asking why. Anyone else seen this? Will update as I hear more.
 

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I had a message that I had a DEF quality issue--took it in to the dealer I purchased it from and the mechanic "cleared" the system. Drove it another 300 miles and the message re-appeared. Took it to another dealer and their, more experienced mechanic replaced the sensor. That took care of the problem. Some of the mechanics are just not comfortable/knowledgeable enough to deal with the DEF system. My original dealer/mechanic called me to find out how the other mechanic "fixed" the problem. I doubt your truck needs a new catalytic converter--but what do I know. Tell them to also replace the DEF sensor.
 

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Something does not make sense. You had a speed restriction message for low DEF. Was the MIL on or the Service Exhaust System indicator on or flashing? What code(s) are stored?
 

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It was over a year ago but I remember the message was "Exhaust Fluid Poor Quality-see owners manual now." The issue is addressed in my 2011 LML manual on page 9-15.

I added fresh fluid and the message continued. I then took it in to the dealer. The original dealer I took it to just "reset" the DIC message center to make the Poor Quality Message go away. That "took care" of the issue for about 300 miles and then the message came back on. The second dealer ( with an older diesel mechanic) replaced the NOx sensor and, since then, I've had no other problems.

So I did not specifically have a speed restriction message--but I was real concerned that I would had I not taken care of the issue. I have no idea what codes appeared when the dealers checked them.

I hope this clarifies my experience.
 
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