Likely culprit EGR cooler. I Think emissions related its covered to 80,000 miles. Keep coolant / distilled water at least to the degas tank seam until it’s replaced. If you are past the 100k powertrain warranty many people get the delete kit from Sasquatch and have their GDE or PPEI tune updated so their is no codes. Cheaper way to fix the problem permanently and take some of the unnecessary cooling load off your cooling system.