The first place to begin is to look at the codes the engine has set. Your engine is an electronic marvel, and nobody can diagnose them without knowing all the malfunction indicator codes the truck has set.
To see the codes, you must have either a scanner or computer software that can access and read the Ford PowerStroke codes set in the power train module (PCM, or main computer). Cheap scanners won't have the power you need to access all the PowerStroke codes. So you need to either take it to a shop that has the correct scanner, or else you need to buy a scanner or software to do the job right.
A reasonable solution - for not much money assuming you have a laptop - is to order AutoEnginuity software, including the extra-cost Ford enhancement. Even the cheapest Windows laptop will work, so if you don't have one, then maybe buy a used one for a coupla hundred bucks.
Most pros use a scanner, but not just any scanner. The Hickok NGS is the most popular:
Hickok, Inc. - For Your Quality Automotive Diagnostic Equipment Needs
Note that Hickok also sells the NGS software without the scanner, so you can also use the best with your laptop. But the NGS software costs a lot more than the AutoEngenuity software, so most shade-tree mechanics go with the AutoEngenuity.
My Sierra Blanca in the sig pic was a great pickup for 11.5 years. I sold it a coupla years ago. I drove a hand-me-down 2003 F-150 SuperCrew 4.6L 2V for a while, but it was unacceptable for towing more than a rowboat. Replacement is a 2012 F-150 EcoBoost SuperCrew Lariat that tows my 5,000-pound TT like a dream.