What are the symptoms you are experiencing (if any, other than the P2138 code of course)? One item you didn't mention having done is to recalibrate the throttle pedal. I would do that before making any other changes.
I would suggest to lookup the procedure for yours as it could be different than what I am sharing here, but the process is this for many Fords.
1) Key on with engine off
2) Wait a few moments for the system to come online and gauges settle down.
3) Press the throttle pedal to the floor in one slow smooth operation (5~10 seconds)
4) Once the pedal is on the floor, let it go (take your foot off of it).
5) Turn off the key and wait ~5 seconds.
Your pedal should now be calibrated. I have read from others that doing this several times helps, others state to only repeat this procedure if the process did not work or the problem (code or otherwise) continues. So, I would do it once, clear the code, and drive it and see if there is any difference, if the code reappears I would repeat the procedure at least 3 times and only after that would I dig deeper into other causes (such as a poor electrical connection to the pedal or PCM module). ~CA
BTW, I just noticed this is your first post so, Welcome to the IRV2 Forums.