If you cant pull codes, check all boots on compressed air intake side to manifold for leaks, popped off boots. Also, any temp issues as these will put in limp mode if to hot, ie rad and water pump working, cooolant ,evel ect. Its also possible turbo sticking closed. Try an italian tune up, leafe the trailer and go for a hard rive, see if you can get boost to 27 psi. A couple hard pulls, accelerate hard wide open and then cool down and do again and again. Also, make sure fuel filters are good, no water in fuel, quality fool before the italian tune up. If the unison ring in turbo is sticking closed, explain low power low boost. If this fails, try an autoparts store to read codes thats close by.