There are 2 error code 13's. One is the 4wd engagement circuit.....it says 4wd is permanently engaged....causes 1. short circuit to ground 2. Solenoid terminals disconnected 3. naked wire. The other is "Hi-Lo P. buttons"....activated when they are pressed simultaneously....causes 1. both pressed simultaneously 2. Lo-P button pressed and high locked 3. Hi-P button pressed and Lo locked....This is a problem with the power shuttle. What year is your tn75s? Mine is an '02 and the manual I have is for 98 to 03 models. I hope this helps. Let us know. I bought the manuals after I had error code 33 for a couple of years....dealership no help. Turned out to be a faulty tilt sensor which was easily identified using the manual.