Before beginning service of the HVAC system, perform the functional test. In order to obtain accurate test results, the thermostat should be completely open.

1. Start the engine, and warm the engine to normal operating temperature.
2. Touch the upper radiator hose. The hose should feel hot, indicating that the thermostat is open.
3. Set the fresh/recirc lever to the FRESH position.
4. Set the temperature lever to the coldest position.
5. Set the blower speed to the HIGH position.
6. Systematically move the mode lever through the lever's various positions. Note which vents expel air at each setting.

In the vent mode, air comes from the instrument panel vents.

In the heat mode, most of the air comes from the floor vents with some air to the windshield.

In the bi-level mode, air should come from the instrument panel vents and from the floor vents.

In the defrost mode, most of the air should go to the windshield with some air going through the floor vent.

7. Ensure that the airflow corresponds with the symbols above each mode lever position. The temperature of incoming air and the temperature of the outside air should be equal.
8. Set the temperature lever to the hottest position. The temperature of the incoming air should be much warmer than the temperature of the outside air.
9. Systematically move the mode lever through the lever's various positions. Note which vents expel air at each setting.
10. Gradually move the blower speed lever from HIGH to LOW. The force of the air from the vents should decrease noticeably. The blower noise at each successive setting should decrease.

If the functional test indicates improper air delivery, or a failure to shift modes, refer to the corresponding diagnostic symptom table.