If you notice that your Nissan Pathfinder is running a bit hot, or that only cool air is blowing when you turn on the heater, chances are you'll have to change the thermostat. Changing the thermostat on a Nissan Pathfinder can, unfortunately, get a bit involved.
Loosen the drain plug at the bottom of the radiator and drain the coolant from your Pathfinder into a suitable container. Reuse the coolant if it is relatively new and clean.
Remove the fan shroud. Follow the upper radiator hose to the intake manifold. Locate the thermostat housing at the point where the radiator hose meets the front of the manifold. Pull off the upper radiator hose from the thermostat housing by loosening the clip at the engine-end of the hose. Remove the lower radiator hose as well.
Take out the two bolts holding the thermostat housing in place. In some Pathfinder models, you may need to remove the drive belts and pulley bracket for easier access to the thermostat housing.
Note the position of the old thermostat. Install the new thermostat into the housing in the same position. Install thermostat gasket. For VG30 engines produced after 1987, Nissan Pathfinders use a liquid gasket rather than a paper gasket. Use a paper gasket for all other Nissan Pathfinder engines.
Reassemble the thermostat housing. Reattach the radiator hose and reinstall the fan shroud. Fill the radiator with reserve or new coolant. Run your Pathfinder with the heater blowing until the engine warms up. Check for leaks.
Do not drain coolant from your Nissan Pathfinder if the engine is hot. Allow the engine to cool before attempting to drain the coolant.