2007-2016 Honda TRX 420 TRX420 Rancher ATV OE Shift Angle Sensor location and tools needed. Easy to replace, especially if the ATV it in neutral. Need a 4mm hex, 10mm wrench, 10mm socket/ratchet and a small screw driver to get the clip loose on the wire harness. I should have videoed the whole process, but wasn't sure this was the problem. Originally the teeth on the shift gears were chewed up, this could have been the cause because from time to time the ATV would just get stuck in a gear and not allow any shifting. Could turn off the ATV and back on and sometimes it would work, sometimes it wouldn't.

BASIC INSTURCTIONS:
1) Use the 10mm socket / ratchet to remove the bottom skid plate
2) Use the screw driver to push up the tab that secures the wire harness, use light upward pressure, then just push a little on each side until it pops off the top of the angle sensor (or until you can grab it with your hand and wiggle off) BE GENTLE and you shouldn't break anything.
3) Use the 4mm hex wrench to remove the two allen bolts from the part. Again, go slow so you don't break anything, the bolts probably have plenty of mud and crud in the threads.
4) Pull out the angle sensor, don't miss the o-ring. It might stay on the part, it might stay on the case. Either way when you reassemble, you want one and only one o-ring.
5) Clean the threads on the bolts, use a little anti-seize on them for "next time" and start every bolt by hand to avoid cross threading.
6) Reassemble in reverse order and DO NOT over tighten. 4mm allen bolts are not all that big and don't require but maybe 10 inch pounds (not foot pounds) of force to keep them in. Think snug and then 1/8 to 1/4 turn extra at the most if you don't have a torque wrench.
7) Personally I'd test the new part now, then if it all works reassemble the skid plate.

Same angle sensor applies to the following years: 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016 (I'm assuming it is in the same location but it might not be. This was a 2007 Honda ES)

Note: there are delete kits for the whole angle sensor, shift computer, etc...but I'd try this first.