EGR valve mechanical problem, usually ash/carbone deposits ore worn actuator. Sometimes clogged DPF with the same exaust/egr flow problems
http://www.ddcsn.com/cps/rde/xbcr/ddcsn/18SP684Rev.pdf https://ddcsn-ddc.freightliner.com/cps/rde/xbcr/ddcsn/08DD159.pdf https://ddcsn-ddc.freightliner.com/cps/rde/xbcr/ddcsn/08ATS2.pdf Read all this before, after do the EGR inspection