Before beginning service of the heater and ventilation system, perform the following functional test. Ensure the thermostat is fully open in order obtain accurate test results.

1. Start the engine.
2. Warm the engine to a normal operating temperature.
3. Touch the upper radiator hose.

A hot radiator hose indicates an open thermostat.

4. Set the fresh/recirc lever to FRESH.
5. Set the temperature lever to the full COLD position.
6. Set the blower speed to HI.
7. Systematically move the mode lever through the various positions.
8. Note which vents expel air at each setting.
9. Verify that the air flow corresponds with the setting of the mode lever.
10. Verify that the air entering the passenger compartment is approximately the same temperature as the air outside of the vehicle.
11. Set the temperature lever to the full HOT position.
12. Verify that the air entering the passenger compartment is much warmer than the air outside of the vehicle.
13. Systematically, move the mode lever to each position.
14. Note which vents expel air at each setting.
15. Gradually move the blower speed from HI to LOW.
16. Verify that the force of the air flow at each setting corresponds with the setting.
17. Verify that the blower noise decreases at each successive setting.

If the functional test indicates improper air delivery or a failure to shift modes when the mode lever settings are changed, refer to the corresponding symptom tables for diagnosis.