RPM Gauge issue

I have a John Deere 3720 a day or 2 ago I was using it and I noticed that the rpm gauge needle was only going down to 20. So I shut the engine off and the needle only went down to 20 if I gave it some rpm its go up to 30 and hit the needle stop. I checked the dash and there were no codes. So I cycled the key a few times and then for a second I saw error 08 which is for the throttle position sensor. But the code did not appear again after that. So I unplugged the sensor by the foot throttle and then plugged back in and the needle went down to 0. But now the needle stops at 15 and will not go any higher. I have no idea what is going on or what I could do to fix it either. Any help would be greatly appreciated.