Honda FOURTRAX RUBICON 2018 Owner's Manual page 166

Coolant
Radiator Core
Refer to Safety Precautions on page 119.
Check the air passages for clogging or damage. Remove insects,
mud, or any obstruction with low water pressure. Have the radiator
checked by your dealer if the air flow is restricted over more than
20% of the radiator surface.
Clean the radiator core after riding the ATV in mud.
The left and centre radiator grill covers must be removed to clean the
radiator core.
156 Servicing Your Honda