Do an operational test of the thermostat using the following procedure:

1. Drain the cooling system. Refer to Cooling System Draining and Filling .
2. Remove the thermostat.

•	For the 4.3L engine refer to Engine Coolant Thermostat Replacement .

•	For the 4.8L, 5.3L, and 6.0L engines, refer to Engine Coolant Thermostat Replacement .




3. Unless the thermostat is obviously faulty, test the thermostat before replacing the thermostat using the following procedure:

3.1.	Place the thermostat and a thermometer in a container with a 50/50 solution of water and DEX-COOL antifreeze.

Important: While heating, do not rest the thermostat or the thermometer on the bottom of the container. This causes the thermostat and the thermometer to be at a higher temperature than the solution.

3.2.	Place the container over a heater.

3.3.	Agitate the solution in order to maintain a uniform temperature of the solution, the thermostat, and the thermometer.

The thermostat valve should start to open at the rated temperature. The thermostat should be fully open after the temperature has increased 15°C (27°F).

A used thermostat can be about 6°C (10°F) above or below these settings without adversely affecting the thermostat. You do not need to replace the thermostat. If the thermostat does not operate at the temperatures specified, replace the thermostat.