Outside Air Temperature Display Inaccurate or Inoperative
Outside Air Temperature Display Inaccurate or Inoperative
Diagnostic Fault Information
Important: Always perform the Diagnostic System Check - Vehicle Initial Inspection and Diagnostic Overview
Circuit/System Description
The HVAC module monitors the signal circuit of the outside air temperature sensor. The outside air temperature (OAT) sensor is a thermistor which varies in resistance as the temperature changes. When the resistance of OAT sensor increases, the outside air temperature is cold. When the resistance of the OAT sensor decreases, the outside air temperature is high. The HVAC module interfaces with the ambient air temperature via two discreet circuits. The HVAC module applies 5 volts to an internal input resistor that is connected to the signal circuit of the outside air temperature sensor. The HVAC module provides ground through the low reference circuit. The HVAC module uses the outside air temperature resistance and voltage for automatic control calculations and conversion to temperature values. The HVAC module sends the temperature value to the instrument panel cluster (IPC) via GMLAN serial data circuits for display in the driver information center (DIC). If the OAT sensor is opened or shorted, the temperature display is either a cold or hot extreme. An open OAT sensor will display -40°F (-40°C) in the DIC and an OAT sensor shorted to ground displays +189.5°F (+87.5°C).
Diagnostic Aids
* The following variable represents the latest temperature value to be displayed by the DIC. This variable can be Degrees Fahrenheit or Celsius.
Range: -40°F to +189.5°F (-40°C to 87.5°F). The default readout is 190°F (87.5°C).
* After the ignition has been shutoff and the engine is not running, the OAT sensor must cool down for a period of time before OAT sensor reading is accurate. This is because the engine produces considerable heat after it has been shutdown. Without air movement across the OAT sensor such as when the vehicle is moving, the OAT sensor provides a false outside temperature reading that can be attributed to engine heat. A falling OAT sensor reading is always accepted and filtered into the display.
* Upon initial vehicle startup, the OAT sensor speed counter estimates the amount of time the vehicle will need to be in motion before the OAT sensor will be reading true outside temperature. As soon as the counter reaches this point it is assumed the driver has been driving the given amount of time for the sensor to cool to ambient. At this point the outside air temperature readings are filtered into the display.
Circuit/System Verification
The DIC should display 189.5°F (87.5°C) or -40°F (-40°C).
Circuit/System Testing
1. Ignition OFF disconnect the harness connector at the outside air temperature (OAT) sensor.
2. Ignition OFF, test for less than 1 ohm of resistance between the ground circuit terminal and ground.
3. Ignition ON, verify the driver information center (DIC) displays approximately -40�F (-40°C).
• If the DIC does not display approximately -40°F (-40°C), test the OAT sensor signal circuit for a short to ground. If the circuit tests normal, replace the HVAC module.
4. Ignition ON, scan tool installed, perform an outside air temperature (OAT) Instant Update to bypass the filtering with either a scan tool or by press and holding the AC and RECIRC buttons simultaneously for 2 seconds.
5. Ignition ON, install a 3-amp fused jumper wire between the signal circuit and the ground circuit of the OAT sensor. Verify the DIC displays approximately 189.5°F (87.5°C).
• If the DIC does not display approximately 189.5°F (87.5°C), test the signal circuit of the OAT sensor for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC module.
6. If all circuits test normal, test or replace the OAT sensor.
Repair Instructions
Important: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests
* Ambient Air Temperature Gage Sensor Replacement
* Programming and Relearning Programming and Relearning