For more information on the Cat C15 coolant flow diagram and the engine's cooling system, refer to the following resources:
Finally, the heated coolant reaches the thermostat housing (water temperature regulator) at the front of the head. Regulation: Radiator vs. Bypass Cat C15 Coolant Flow Diagram
The keyword "C15" covers three distinct generations. Do not use a 6NZ diagram for a C15 Twin-Turbo (CT15). For more information on the Cat C15 coolant