Honda TRX500FA6 AT Owner's Manual page 170

Coolant
3. Remove the radiator cap lid (4) by disengaging the stud (5) from
the grommet (6).
4. Remove the reserve tank cap (7).
Always add coolant to the reserve tank. Do not attempt to add
coolant by removing the radiator cap.
5. Add coolant to the reserve tank as required to bring the coolant
level to the UPPER level mark.
FRONT
(4) radiator cap lid
(5) stud
6. After adding coolant, reinstall the reserve tank cap.
7. Install the radiator cap lid.
160 Servicing Your Honda
(5)
(6)
(4)
(7)
(6) grommet
(7) reserve tank cap