Did they change both filters? If not not get the primary filter changed. It if is plugged it will not flow. I would get the CAT 1R-0751 installed ASAP. CATs don't like dirty fuel.
The C7 uses HEUI (Hydraulic/Electronic Unit Injector) system. I would google it and read about it. This is a very complicated system. It used engine oil to inject the fuel. Dirty or old engine oil can cause issues. The HEUI increases the oil pressure in the pump from 500 to 3600 PSI depending load. That oil is use in the injector to push the "intensifier piston" to inject fuel. The intensifier piston is a 7 to 1 ratio. So fuel going into the injector at 1000PSI gets injected in to then cylinder at 7000PSI. There is a lot of wear and tear on the oil.
Clean fuel and oil are you friends.
