Before beginning service of the heater and ventilation system, perform the functional test. In order to obtain accurate test results, ensure that the thermostat is fully open.

1. Start the engine and warm-up to normal operating temperature.
2. Touch the upper radiator hose. The hose should feel hot to the touch, indicating that the thermostat is open.
3. Set the fresh/recirc lever to FRESH.
4. Set the temperature lever to full COLD (left).
5. Set the blower speed to HI.
6. Systematically move the mode lever through the lever's various positions noting from which vents the air is expelled at each setting.

In the bi-level mode, air should come from the IP vents and from the floor. In the defroster mode, most of the air should be directed at the windshield with some air directed at the floor vent, and so on.

7. Ensure that the points where the air is directed correspond with the symbols above each mode lever position. The air coming into the passenger compartment should be approximately the same temperature as the temperature outside the vehicle.
8. Set the temperature lever to full HOT (right). The temperature of the air entering the passenger compartment should now be much warmer than the air outside the vehicle.
9. Systematically move the mode lever through the lever's various positions noting from which vents the air is expelled at each setting.
10. Gradually move the blower speed lever from HI to LOW. There should be a noticeable decrease in the forcefulness with which the air is expelled from the vents and a decrease in the blower noise at each successive setting.

If the functional test indicates improper air delivery or a failure to shift modes when the mode lever is moved, refer to the corresponding diagnostic symptom table(s).