I see you're from Australia. I can only offer what I know on trucks made for the US market. The 816 is not offered for the US market.
But just for the hell of it...
Your cruise control switch, does it have a push button and a toggle that moves up and down? If so, hold the button (Mode switch) in until a sub menu pops up. What are the options? Over here, one would tap the toggle up to scroll through the menu items. Scroll up until 'SYSTEM CHECK' is highlighted. Tap down on the toggle to select.
Active codes show up. toggle up to see them all. Now hold the toggle up for several seconds. Inactive codes should appear. Write them all down before deleting them! Actually, don't delete them at all, it won't fix anything, it'll just make troubleshooting harder (since they're all date stamped). You can hold the toggle down for several seconds to delete them, but it won't help you.
After you get the codes, post them here along with whether the're active or inactive.
Does it have a DPF? Is there soot in the tailpipe? Does it have a steel fuel tank and there's rust chunks in the bottom? Does the check engine light go out after the engine is shut off and is then restarted, or is it on all the time? There are numerous reasons for limp mode, but some are much more common than others.