Circuit Description
The HVAC control module monitors the upper left and upper right duct temperature sensors when the mode switch is in the VENT or BI-LEVEL positions. The HVAC control module monitors the lower left and lower right duct temperature sensors when the mode switch is in the DEFOG, FLOOR or FRONT DEFROST positions. The duct sensors are used to measure the temperature of the air exiting the ducts to help maintain a consistent cabin air temperature. These duct sensor inputs provide the HVAC control module a way to determine if the engine needs to be started to maintain a specific air temperature. If an upper duct sensor circuit failure is detected the HVAC module will not request an engine start to help cool the cabin. A lower duct sensor circuit failure will not prevent the engine from starting.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC B0990 Output Air Temperature Upper Sensors Correlation
Conditions for Running the DTC
The ignition has been in the OFF position for 4 hours.
Conditions for Setting the DTC
| • | After a 4-hour ignition OFF the difference of lower air temperature sensors count value is more than 40 counts . |
| • | After engine has started, within the first 20 minutes, and the conditions are present for the HVAC control module to place the air temperature doors to the maximum heat position and the air temperature sensor count value does not change at least 50 counts. |
| • | If the previous condition has passed a second 20-minute timer is started and conditions are present for maximum heat and the air temperature sensors are more than 40 counts apart any time during this second 20-minute timer. |
Action Taken When the DTC Sets
| • | The control module stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The driver information center, if equipped, may display a message. |
| • | The HVAC control module will not request an engine restart while in continuous mode. |
Conditions for Clearing the DTC
| • | A current DTC Last Test Failed clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Diagnostic Aids
| • | If the condition is intermittent, refer to Testing for Intermittent Conditions and Poor Connections . |
| • | An air temperature sensor not installed properly in the HVAC duct may cause this DTC. |
| • | DTC B0174, B0179, B0510 and B0515 must be repaired before this diagnostic is completed or misdiagnosis will occur. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step tests to ensure that an air temperature sensor DTC did not cause this DTC.
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DTC B0989 sets only for the lower air temperature sensors. The HVAC system is placed in the Bi-Level mode and set to the coldest position to obtain a consistent temperature in the HVAC ducts. This action will determine if one of the air temperature sensors reading may be slightly higher or lower than the other air temperature sensors.
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The appropriate air temperature sensor is the sensor that is out of range compared to the other duct air temperature sensors.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: HVAC Schematics Connector End View Reference: HVAC Connector End Views | |||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||||
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Important: DTC B0174, B0179, B0510 and B0515 must be repaired before this diagnostic is completed or misdiagnosis will occur.
Does the scan tool indicate that DTC B0174, B0179, B0510 and B0515 are current? | Go to Step 3 | ||||||||||
3 | With a scan tool, monitor the DTC Information for DTC B0990 in the HVAC control module. Does the scan tool indicate that DTC B0990 is current? | Go to Step 4 | Go to Diagnostic Aids | ||||||||
4 |
Are the RH Heater Duct Actual and the LH Heater Duct Actual parameters within 30 counts of each other? | Go to Diagnostic Aids | Go to Step 5 | ||||||||
Is one the AC Duct Actual parameters not within 30 counts of the other air temperature sensors count value? | Go to Step 6 | Go to Diagnostic Aids | |||||||||
Inspect the appropriate air temperature sensor for proper installation. Refer to the appropriate procedure: Did you find and correct the condition? | Go to Step 14 | Go to Step 7 | |||||||||
7 |
Does the voltage measure 4.9 volts or more? | Go to Step 8 | Go to Step 10 | ||||||||
8 | Test the signal circuit of the appropriate air temperature sensor for the following:
Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 14 | Go to Step 9 | ||||||||
9 | Test the low reference circuit of the appropriate air temperature sensor for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 14 | Go to Step 11 | ||||||||
10 | Inspect for poor connections at the harness connector of the HVAC control module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 14 | Go to Step 13 | ||||||||
11 | Inspect for poor connections at the harness connector of the appropriate air temperature sensor assembly. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 14 | Go to Step 12 | ||||||||
12 | Replace the air temperature sensor assembly. Refer to the appropriate replacement procedure: Did you complete the replacement? | Go to Step 14 | -- | ||||||||
13 | Replace the HVAC control module. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 14 | -- | ||||||||
14 |
Does the DTC reset? | Go to Step 2 | System OK | ||||||||
