Fixing your DPF Problems in a nutshell...

[Waterloo]

How many miles on the truck? Make and Model? Do you know about the fuel pumps? Any codes?

What prompted this adventure leading you to replace the EGR Cooler? Dealership visit, if so what did you take the truck in for?

Were you told WHY the EGR Cooler needed to be replaced? Those things are darn near bulletproof, very rare for one to fail, especially one as new as yours. Again, sounds like you were at a dealership... If you were, they are changing their game, usually they trick you into replacing your turbo first, then nail you for the EGR Cooler...

Have you removed the exhaust at the turbo, start the truck and see what happens, sounds like a restriction somewhere, or carbon packed.

When was the last time the DPF and DOC cans were serviced?

Has the SCR system been serviced, new filter? If so, when?

How is the coolant and oil levels, water in the block, oil in coolant, hydro locked?

Have you tried to bar the motor?

How about the overhead, the valves? Was anyone in there?

I'm thinking carbon packed, pull the exhaust at the turbo and fire her up and see if you notice a difference.