B0408

DTC B0408 Temperature Control #1 (Main/Front) Circuit Malfunction






Circuit Description

The HVAC control module receives a position signal from the air temperature actuator. The air temperature actuator position sensor is a potentiometer that is tied to a 5 V reference source. The variable output from the potentiometer feeds back a position signal to a pull-up resistor which is located in the HVAC control module. This signal determines the necessary drive signal to the actuator motor which provides the proper air temperature door position. This value is stored in the Keep Alive Memory (KAM) portion of the HVAC control module. When the air temperature door is in a full hot position the scan tool will read above 190 Counts. When the air temperature door is in full cold position the scan tool will read below 48 Counts. Normal operating range is 46 Counts to 194 Counts.

Conditions for Running the DTC

The ignition is in the ON position.

Conditions for Setting the DTC

The HVAC control module detects this condition when the ignition switch is in the ON position and one or more of the following conditions exists:

- Ignition 3 voltage is open.
- The air temperature actuator signal circuit is shorted to voltage, ground, or open.
- The 5 V air temperature door control circuit is shorted to voltage, ground, or open.
- The 5 V reference is shorted to voltage, ground, or open.
- Low reference is open.

The HVAC control module does not sense the expected change in the temperature door position when the air temperature valve sensor and motor is commanded to move the temperature valve.

Action Taken When the DTC Sets

The HVAC control module drives the temperature valve to the full COLD position when the system is in one of the following modes:

- Panel
- Bi-level
- Auto Panel/Auto Bi-level

The HVAC control module drives the temperature valve to full HEAT position when the system is in one of the following modes:

- Lower
- Defog
- Defrost
- Auto Lower

Conditions for Clearing the DTC

- The DTC becomes history when the HVAC control module no longer detects a failure.
- The history DTC clears after 50 fault-free ignition cycles.
- The DTC can be cleared with a scan tool.

Diagnostic Aids

- Normal operating range is 40 Counts to 200 Counts. If out of this range check for binding, misalignment, or broken air temperature door.
- If condition is not present refer to Testing for Intermittent Conditions and Poor Connections.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

2. Test for other DTC that are current DTC that could also set this DTC but would need to be diagnosed first.
3. Tests for the proper operation of the circuit to insure that commanded and actual positions are within specifications.
4. Tests for the proper operation of the circuit when door is commanded to move.