For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC P0856: Traction Control Torque Request Signal Message Counter Incorrect

### Circuit/System Description

The electronic brake control module (EBCM) and the engine control module (ECM) simultaneously control the traction control. The EBCM sends a serial data message to the ECM requesting torque reduction. When certain ECM DTCs are set, the ECM will not be able to perform the torque reduction for traction control. A serial data message is sent to the EBCM indicating that traction control is not allowed.

### Conditions for Running the DTC

• | Ignition ON.

• | Ignition voltage is greater than 10 volts.

### Conditions for Setting the DTC

The ECM detects a fault that prevents it from performing a traction control function and sends a serial data message to the EBCM indicating that torque reduction is not allowed.

### Action Taken When the DTC Sets

• | The EBCM disables the TCS for the duration of the ignition cycle.

• | The ABS remains functional.

• | A DIC message and/or a warning indicator may be displayed.

### Conditions for Clearing the DTC

• | The condition for the DTC is no longer present.

• | The electronic brake control module (EBCM) clears the history DTC when a current DTC is not detected in 100 consecutive drive cycles.

### Diagnostic Aids

These DTCs are an indicator that the ECM receives an invalid message from the EBCM , and does not necessarily indicate a fault is present with the EBCM. This DTC should not be used as a fault indicator, but rather as a symptom to assist in the diagnosis of these other systems.

### Reference Information

#### Schematic Reference

Antilock Brake System Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

ABS Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

Verify that no DTCs are set in the ECM.

⇒ | If DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.