HVAC System - Manual

DTC B0408 or B0423

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC B0408 02
- Temperature Control 1 Feedback Circuit Short to Ground

DTC B0408 05
- Temperature Control 1 Feedback Circuit Short to Battery or Open

DTC B0408 61
- Temperature Control 1 Feedback Circuit Actuator Stuck Open

Diagnostic Fault Information






Circuit/System Description

The HVAC control module controls the HVAC door actuator to regulate the airflow through the HVAC system. The actuator consists of an electric motor and a potentiometer. The module supplies a low reference and 5-volt reference source voltage to the potentiometer. The HVAC control module monitors the voltage drop across the potentiometer on the door position signal circuit. When the actuator shaft rotates, the voltage on the door position signal circuit changes. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions.

DTC B0408 is for the drivers air temperature actuator.

Conditions for Running the DTC

* The ignition is ON.
* Ignition voltage is between 9-16 volts.
* The HVAC module is ON.

Conditions for Setting the DTC

The actual door position differs from the commanded door position by more than 4 counts or the HVAC control module detects the door position signal circuit is less than 7 counts or greater than 250 counts.

Action Taken When the DTC Sets

The control circuit is deactivated for the appropriate actuator.

Conditions for Clearing the DTC

* The DTC becomes history when the HVAC control module no longer detects the condition that set the DTC.
* The history DTC will clear after 50 fault-free ignition cycles.
* The DTC can be cleared with a scan tool.

Circuit/System Verification

Ignition ON, command the actuator to 100 percent with a scan tool. The scan tool should display between 224 and 230 counts on the mode actuator, all other actuators should display between 199 and 205 counts.

Circuit/System Testing

1. Ignition OFF, disconnect the recirc actuator harness connector at the actuator.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the low reference circuit terminal 1and ground.

• If greater than 1.0 ohm, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the HVAC control module.

3. Ignition ON, test for 4.8 to 5.2 volts between the 5-volt reference circuit terminal 3 and ground.

• If less than 4.8 volts, test the 5-volt reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

• If greater than 5.2 volts, test the 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the HVAC control module.

4. Verify the appropriate scan tool parameter is less than 3 counts.

• If greater than the specified range, test the signal circuit terminal 2 for a short to ground. If the circuit tests normal, replace the HVAC control module.

5. Install a 3A fused jumper wire between the signal circuit terminal 2 and the low reference circuit terminal 1. Verify the appropriate scan tool actuator parameter is greater than 250 counts.

• If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

6. Connect a test lamp between control circuit A terminal 5 and control circuit B terminal 6.
7. Command the actuator in both directions with a scan tool. The test lamp should turn ON and OFF with each command.

• If the test lamp remains always ON or always OFF during either commands, test for an open, short to ground, or short to voltage on either control circuit. If the circuit tests normal, replace the HVAC control module.

8. If all circuits test normal, replace the actuator.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

* Refer to Programming and Relearning for HVAC replacement, setup, and programming
* Air Temperature Actuator Replacement - Right Side Verification Tests