Before beginning the function test of the heater system, idle the vehicle with the coolant hot and the thermostat open. The coolant temperature should be approximately 90°C (194°F). During the function test, evaluate the operating efforts of the mode rotary knob and the temperature rotary knob. If you find a condition during any intermediate step of the function test, complete the test before starting the repair. Use the Tech II in order to verify the temperature.

1. Cycle the temperature knob to the extreme ends of travel on the control assembly dial.

Verify that the cable is properly adjusted.

2. Move the mode rotary knob to the VENT position.

•	Air should come from the instrument panel outlets at nearly the same temperature as the outside air.

•	Air should not come from the following outlets:

-	The floor outlets

-	The defroster nozzles

-	The side window defogger outlets

3. Move the mode rotary knob to the HEATER position.

•	Most of the air should come from the floor outlets.

•	Air should not come from the instrument panel outlets.

•	The remainder of the air should come out of the defroster nozzles and the defogger outlets.

4. Move the rotary knob from the HEATER position to the DEFROST position.

•	Most of the air should be discharged onto the windshield from the defroster nozzles.

•	A small amount of air should be discharged from the floor outlets.

5. Move the temperature rotary knob back to the full COLD position.
6. Ensure that the air temperature returns to approximately the temperature of the outside air.
7. Slowly turn the blower switch toward OFF. Stop briefly at each intermediate blower speed position.

•	Note the force of the airflow coming from the instrument panel center outlets.

•	The airflow should reduce noticeably at each blower speed position.

•	Note the sound of the blower.

•	The blower noise should reduce noticeably at each blower speed position.