Truck in limp mode! Help!

[Silverwolf]

Ok, Its a 2006 Volvo Vnl single axle. Has a 2008 Cummins ISX 400St motor, Eaton 10spd lightning shifter. Don't know what de mandated means. But its totally stock, has egr but no def. Never had an issue other than the fuel pressure sensor that set a too high fuel pressure code and limp mode. New sensor fixed that. Previous owner had a new egr valve installed at a hefty price. Doesn't get much mileage as its a personal vehicle used for personal use only. But it does have tons of mileage on it. 1.9million km. Very clean, regularly maintained as in oil changes and regular maintenance. Been very reliable.