For 1990-2009 cars

## DTC P0856 3.6L (LY7)

### Circuit Description

The electronic brake control module (EBCM) and the engine control module (ECM) simultaneously control the traction control. The ECM reduces the amount of torque supplied to the drive wheels by retarding spark timing and selectively turning OFF fuel injectors. The EBCM actively applies the brakes to the front wheels in order to reduce torque.

When certain ECM DTCs are set, the ECM will not be able to perform the torque reduction portion of traction control. A serial data message is sent to the EBCM indicating that traction control is not allowed.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0856 Traction Control System Input Signal

### Conditions for Running the DTC

• | Control Area Network Bus has been initialized and ready.

• | The ignition is on for more than 3 seconds.

• | The ignition 1 voltage is between 10.5-18 volts.

• | Bus Communication runs OK.

• | DTC P0856 runs continuously once the above conditions are met for more than 1 second.

### Conditions for Setting the DTC

• | The traction information which the ECM sends to the EBCM is temporarily stopped or the content of the information is invalid.

• | The above conditions exist for up to 4 seconds.

### Action Taken when the DTC Sets

• | The EBCM disables the EDC/TCS during the ignition cycle.

• | The Traction Off indicator turns ON.

• | The ABS remains functional.

### Conditions for Clearing the DTC

• | The condition for setting the DTC removes and use the scan tool Clear DTCs function to remove the DTCs. The EBCM automatically clears the history DTC when a current DTC is not detected in 100 consecutive drive cycles.

• | The ECM automatically clears the history DTC when a current DTC is not detected in 40 consecutive warm-up cycles.

### Diagnostic Aids

The following conditions can cause this concern:

• | A communication signal frequency problem

• | The EBCM is not receiving information from the ECM.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Install a scan tool.
2. Start the engine.
3. With a scan tool, observe the Requested Torque Signal parameter in the Engine Control Module (ECM) data list.

Does the scan display less than the specified value?

100%

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3 |

1. Turn OFF the ignition.
2. Disconnect the engine control module (ECM) harness connector.
3. Test the requested torque signal circuit for the following conditions:

• | Shorted to Voltage

• | Short to Ground

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition.

--

Go to Step 9

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Disconnect the engine control module (ECM) harness connector.
3. Test the requested torque signal circuit for the following conditions:

• | Open

• | High Resistance

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 5

5 | Inspect for poor connections at the harness connector of the ECM. Refer to Testing for Intermittent Conditions and Poor Connections . Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 6
6 | Inspect for poor connections at the harness connector of the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections . Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 7
7 | Replace the EBCM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | -- | Go to Step 9 | --
8 | Replace the EBCM Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | -- | Go to Step 9 | --
9 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle at the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK

## DTC P0856 4.6 (LH2)

### Circuit Description

The electronic brake control module (EBCM) and the engine control module (ECM) simultaneously control the traction control. The ECM reduces the amount of torque supplied to the drive wheels by retarding spark timing and selectively turning OFF fuel injectors. The EBCM actively applies the brakes to the front wheels in order to reduce torque.

When certain ECM DTCs are set, the ECM will not be able to perform the torque reduction portion of traction control. A serial data message is sent to the EBCM indicating that traction control is not allowed.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0856 Traction Control System Input Signal

### Conditions for Running the DTC

• | Torque reduction signal diagnosis has activated.

• | Engine has been running.

• | Power mode has been activated.

• | Traction Control System sends data information to GM Local Area Network.

• | DTC P0856 runs continuously once the above conditions are met.

### Conditions for Setting the DTC

The traction information which the ECM sends to the EBCM is temporarily stopped or the content of the information is invalid.

### Action Taken when the DTC Sets

• | The EBCM disables the EDC/TCS during the ignition cycle.

• | The Traction Off indicator turns ON.

• | The ABS remains functional.

• | DTC P0856 is a type C DTC.

### Conditions for Clearing the DTC

DTC P0856 is a type C DTC.

### Diagnostic Aids

The following conditions can cause this concern:

• | A communication signal frequency problem.

• | The EBCM is not receiving information from the ECM.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Install a scan tool.
2. Start the engine.
3. With a scan tool, observe the Requested Torque Signal parameter in the Engine Control Module (ECM) data list.

Does the scan display less than the specified value?

100%

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3 |

1. Turn OFF the ignition.
2. Disconnect the engine control module (ECM) harness connector.
3. Test the requested torque signal circuit for the following conditions:

• | Shorted to Voltage

• | Short to Ground

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Disconnect the engine control module (ECM) harness connector.
3. Test the requested torque signal circuit for the following conditions:

• | Open

• | High Resistance

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 5

5 | Inspect for poor connections at the harness connector of the ECM. Refer to Testing for Intermittent Conditions and Poor Connections . | -- | Go to Step 9 | Go to Step 6
6 | Inspect for poor connections at the harness connector of the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections . Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 6
7 | Replace the ECM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | -- | Go to Step 9 | --
8 | Replace the EBCM Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | -- | Go to Step 9 | --
9 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle at the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK