Assuming your F350 does not have an emission systems tamper, in other words, non of the emission components have every been modified and the truck is in the same emissions configuration as when it left the factory; with 49-state legal emissions system (as opposed to 50-State California legal system), and it fails the California smog check, then we have to assume it failed the smog inspection due to damaged/defective emissions component/s and/or high emission levels.
In either case the truck will need repairs in order to pass the smog check. You'll need to have a smog check repair station conduct a hands-on diagnosis in order to reveal the cause of the emissions fault, and have the fault/s repaired.
No engine modification or component additions are required to smog check in California. An out-of-state vehicle can be smog tested and registered in California as long as its emissions systems have not been modified or tampered.
