Before beginning service of the heater and ventilation system, perform this functional test. To obtain accurate test results, the thermostat should be fully open.

1. Start the engine and warm-up to normal operating temperature.
2. Touch the upper radiator hose. It should feel hot to the touch, indicating that the thermostat is open.
3. Set the fresh/recirc lever to the FRESH position.
4. Set the temperature control to the coldest position.
5. Set the blower speed to HIGH.
6. Systematically move the mode control dial through its various positions, noting from which vents the air is expelled at each setting. In the vent mode, air should come from the instrument panel vents.

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

In the heat mode, air should come from the floor vents.

In the heat/defroster mode, most of the air should come from at the windshield vents, with some air from at the floor vents.

In the defroster mode, air should come from the windshield vents.

7. Make sure that the points where the air is being 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 control dial to the hottest position. 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 control dial through its various positions, noting from which vents the air is expelled at each setting.
10. Move the blower speed lever from HIGH gradually down to LOW. There should be a noticeable decrease in the forcefulness with which the air is expelled from the vents, and a decrease in blower noise at each successive setting.

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