Honda FourTrax Rancher TRX420FA5 2019 Owner's Manual page 181

Brakes
Brake Light Switch Adjustment
Refer to Safety Precautions on page 107.
Rear Brake Only:
Check the operation of the rear brake light switch (1) at the right
side behind the engine from time to time. Adjustment is done by
turning the adjusting nut (2). Hold the rear brake light switch and
turn the nut in direction (A) if the switch operates too late and in
direction (B) if the switch operates too soon.
RIGHT REAR
(1)
(A)
(2)
(B)
(1) rear brake light switch (2) adjusting nut
Servicing Your Honda 171