I’m having the same problem here as well, I have a 2007 TRX420 ES model, my grandpa (passed away 3 years ago) had it and passed it down to me, it has ran well over the years, it’s been through a lot, the ES motor isn’t water/mud friendly I’ll tell you that lol. The rancher started having very little bit of shifting problems, when anyone tried to run and drive it all the way to 5th gear, you couldn’t shift back down sometimes so you had to stop and turn the rancher off and back on to get it to reset back to neutral or just rock it back and forth to get it back to lower gear.. The rancher got stuck in neutral one day when we were cutting grass, we just stopped fooling with it, we rolled it and parked it close to our shed, and it’s sat there ever since my grandpa passed.. I’m just now getting around to fix it and so far, it’s been a little rough, 2 gear pieces were chipped off from the gear’s teeth, and to be fair, inside of ES motor was decently clean, not enough grease that I know of, the bearings are surprisingly still in good shape, and I recently got a new battery for it. The only problem I’m having is the manual emergency shifter isn’t shifting up or down, I’ve tried to rock it back and forth but no luck at all, unless I’m not trying hard enough haha. I’ve only been going by some youtube tutorials and it’s helped me a ton because I barely know anything about ES 4 wheelers.. The only question I have is, is it safe to start and let the 4 wheeler run with the ES motor and angle sensor disconnected? I know it’s kinda a dumb question for me to ask but just wondering lol, but from some videos I’ve searched though, I forget what video it was but they say once you disassemble the ES motor and all, make sure to clean and use sandpaper (600 grit might work well) if you need to, use degreaser and let it set about 10-15 minutes and put a small wire brush to it, this process is only for inside of the ES covering. My best bet is to replace the gears, if the little teeth of one gear is broken, then replace both of the gears, one of them almost looks like a shape of a foot and one is round. The teeth of the gear rod that sticks out of the ES motor was broken so I had to buy a new ES motor, but once you get all of the new gears and parts, I heard you have to apply white lithium grease to the gears I’m pretty sure, and apply dielectric grease to the angle sensor, and make sure everything’s aligned correctly, then I think you should be good to go! I hope all works out!