one rule of thumb to remember on these trucks and any in the future that have this type is system on it, when you unplug or change out any part related to the fuel or sensor system it will set a code in the ECM wether the truck is running or not. The only way around this might be to disconnect the battery before doing any sort of work. This is not a common practice umong cat tech's though, they have the diagnozer program and can access the system and erase the codes set after they finish the repairs, also it is required to run certain tests to relearn air fuel mixture ratios stored in the ecm and run a fast tas test.

you can try to do the 'pedal dance' and clear any codes that might be stored but not showing on the display and see if that puts the truck back in normal mode, out of the limp mode it goes in when there is a code set.
Just do a forum search on 'pedal dance' , it has been posted here many times, that should show you what you need to do to access the ecm and attempt to erase the codes.

Note: if the pedal dance does not clear the problem you have something else wrong that needs to be found and addressed. If there is somethine else faulting causing it to be in limp mode and it is not showing an error code would be rare, these truck code out on almost everything that goes wrong ;o)