Most likely the dpfe sensor which measures the amount of egr flowhas gone bad thats the most common problem. Or the EVR egr vaccum regulator isn't working, if you have access to a scan tool you can command the evr on and watch the dpfe pid to see if the voltage changes, when you turn on the evr the truck should idle poorly at the same time, if the idle doesnt change,the evr is not working if it does and the dpfe voltage doesn't change dpfe is bad.