Given that you have posted in the Gen IV area - I'm assuming this is a 2007+ truck given the DTC. This is part of a diagnostic algorithm for catalytic converter warm up from a cold start. Lots of possible contributors - often a bad or aging O2 sensor is at fault.
In older gas calibrations this was an EGR related DTC. In Gen IV engines (no EGR) P1400 is sometimes labeled "Cold start emission reduction control system" ... i.e. cat warm up.
Given the way the vehicle has been configured, just turn off the DTC.