Broad-sweeping computer controls were a fairly new thing at the turn of the millennium -- and the modular engine was even newer to the F-150 line. Codes P1401 and P1518, more than anything else, tend to pop up as reminders that "brand-new" parts aren't always necessarily "improved."
Code P1518 indicates that the intake manifold runner control is stuck open, and P1401 indicates a fault with the "DPFE" sensor that monitors the exhaust gas recirculation system. More than likely, this is a case of two separate faults occurring simultaneously because of age and other issues -- but maybe not.
The DPFE is a plastic sensor that sits in the EGR channel that sends hot exhaust gases to your intake manifold. It double-checks the EGR valve to make sure the gases are flowing at the correct rate. You don't have to be an engineer to know that plastic sensors won't last long sitting in 500-degree exhaust gases -- and indeed, DPFE sensor failures are a well-known problem with Fords of this vintage. Ford issued technical service bulletin 4111, and under Ford program 02M01, repair might be covered for free.
The second code regarding the IMRC has a habit of popping up along with the notorious DPFE code. Your engine actually has two sets of intake runners and two intake ports for each cylinder; a valve opens the second one at high rpm to boost top-end horsepower. The linkage systems, actuator and bushings for these valves will wear out over time, causing them to hang open or closed. Most often, it's the actuator gone bad. It's possible that the DPFE failure had something to do with the failure of the IMRC system, particularly if you've been driving it with a fried DPFE for a while. More than likely, though, your DPFE failure just hastened the demise of an already weak or failing component in the IMRC.
