Guy test rode my 420es and brought it back stuck in 5th. Code showed bad shift angle sensor. The atv also had a cheap es motor so I changed it also along with shift buttons complete with harness as they showed deterioration. I cleaned the gears, changed the 84 tooth? gear as it had two chipped teeth. New lithium grease, back together and up shifts and downshifts fine but no reverse. Removed the es motor and gears and still won’t shift into reverse manually. Something simple.... I think I know the answer😬

 

Right, well you haven't got a 2001 420 as they didn't come out until 06 and, if it won't shift into reverse manually, the lock-out lever at the back of the gearbox isn't moving far enough, probably due to the reverse cable needing adjustment. Often happens after you adjust the handbrake, as the handlebar lever doesn't move as far, neither does the reverse lever. Or, as you have been poking about at the back of the gearbox to fit the new angle sensor, you may have got something wrong when re-fitting the shroud.