Heater System: Diagnosis GMC Envoy I

FUNCTIONAL TEST

1. Idle engine for about 20 minutes until warm or until thermostat opens. Ensure coolant temperature is about 194°F (90°C). Select vent mode. Move temperature lever to coldest temperature setting. Select high blower speed. Air should exit instrument panel outlets and should be about the same temperature as outside air. Air should not exit floor outlets, defrost nozzles, or side window defogger outlets.
2. Select heater mode. Move temperature lever to hottest temperature setting. Most air should exit floor outlets, with remaining air exiting defrost and window defogger outlets and should be hot. Air should not exit instrument panel outlets.
3. Select defrost mode. Most air should exit defrost and defogger outlets, with a low volume of air exiting floor outlets. Move temperature lever to coldest setting. Air temperature should decrease to about the same temperature as outside air.
4. Slowly turn blower motor speed control knob toward OFF position, stopping briefly at each intermediate position to check force of airflow exiting instrument panel center outlets to hear blower noise. Airflow and blower noise should decrease noticeably at each intermediate position.