Thermostat And Thermostat Housing: Inspection

WARNING: This page is about a different car, the 2002 Nissan Sentra. However, it is still accessible from the selected car via links, so may be relevant.

Check for valve seating condition at normal room temperature. The valve should seat tightly all the way around. If the valve is warped or stuck open, replace the thermostat.
Suspend the thermostat, by a string caught in the closed valve, in boiling water while monitoring the temperature.

Fig 1: Thermostat Specifications
Courtesy of NISSAN MOTOR CO., U.S.A.

Fig 2: Testing Thermostat In Hot Water
Courtesy of NISSAN MOTOR CO., U.S.A.
Check the temperature at which the valve begins to open and falls from the string. Check the total valve lift when the valve opens completely.
Then check if valve closes at 5°C (41°F) below valve opening temperature.