For 1990-2009 cars

### Test Description

The number below refers to the step number on the diagnostic table.

1. Command both the ON and OFF states of the rear window defogger switch. Repeat the command as necessary to verify the Rear Defog Switch Input data parameter does or doesn't change states.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Defogger Schematics . Connector End View Reference: Component Connector End Views .
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle .
2 |

1. Turn ON the ignition, with the engine OFF.
2. Observe the rear window defogger indicator on the HVAC control assembly.
3. Command the rear window defogger indicator ON and OFF, by depressing the rear window defogger switch.

Does the rear window defogger indicator turn ON and OFF with each command?

Go to Testing for Intermittent Conditions and Poor Connections .

Go to Step 3

3 |

1. Install a scan tool.
2. With the scan tool, observe the Rear Defog Switch Input Data parameter in the body control module (BCM) HVAC data list.
3. Depress the rear window defogger switch.

Does the Rear Defog Switch Input Data parameter always display On?

Go to Step 4

Go to Step 5

4 |

1. Turn OFF the ignition.
2. Disconnect harness connector xx of the BCM.
3. Turn ON the ignition, with the engine OFF.
4. Connect a test lamp from the rear window defogger switch signal circuit to battery voltage.

Does the test lamp illuminate?

Go to Step 6

Go to Step 7

5 |

1. Turn OFF the ignition.
2. Disconnect harness connector xx of the BCM.
3. Turn ON the ignition, with the engine OFF.
4. Connect a test lamp from the supply voltage circuit of the REAR DEFOG relay coil to a good ground.

Does the test lamp illuminate?

Go to Step 9

Go to Step 10

6 | Test the rear defogger switch signal circuit of the HVAC control assembly for short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 8
7 | Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 11
8 | Inspect for poor connections at the harness connector of the HVAC control assembly. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 12
9 | Repair a short to voltage in the supply voltage circuit of the REAR DEFOG relay coil. Refer to Wiring Repairs . Did you complete the repair? | Go to Step 13 | --
10 | Repair a short to voltage in the supply voltage circuit of the rear window defogger indicator. Refer to Wiring Repairs . Did you complete the repair? | Go to Step 13 | --
11 | Replace the BCM. Refer to Control Module References . Did you complete the replacement? | Go to Step 13 | --
12 | Replace the HVAC control assembly. Refer to HVAC Control Module Replacement . Did you complete the replacement? | Go to Step 13 | --
13 | Operate the system in order to verify the repair. Did you correct the condition? | System OK | Go to Step 2