For 1990-2009 cars

### Circuit Description

The controller area network (CAN) serial data line is a high speed serial data bus used to communicate information between the engine control module (ECM), body control module (BCM), and the transmission control module (TCM). Typical data-transmission speeds must be high enough to ensure that the required real-time response is maintained. The CAN serial data line does not communicate with the scan tool via the data link connector (DLC). CAN serial data information is interpreted by the BCM and transmitted to the serial data line by the BCM.

Modules connected to the CAN serial data circuit monitors communication during normal vehicle operation, where operation information is exchanged among the modules. Each module on the CAN network maintains a transmit error counter (TEC) and a receive error counter (REC). The counter values increase with detected errors and will decrease with error-free messages. If the TEC value exceeds 255, the controller removes itself for the network and DTC U2100 will result.

### Conditions for Running the DTC

• | Supply voltage to the modules that are in the normal operating range.

• | The vehicle is in the RUN power mode.

### Conditions for Setting the DTC

The module setting the DTC has attempted to establish communications on the CAN circuits for more than 7 times.

### Action Taken When the DTC Sets

• | The ECM turns ON the malfunction indicator lamp (MIL) during the second consecutive drive cycle with the error detected.

• | The ECM records the operating conditions at the time of turning ON the MIL and stores the data as Freeze Frame information.

### Conditions for Clearing the MIL/DTC

• | A current DTC Last Test Failed clears when the diagnostic runs and passes after 3 consecutive drive cycles.

• | A current DTC clears and the MIL is turned OFF when the fault is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the fault, or after 40 consecutive drive cycles.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Link Connector Schematics Connector End View Reference: Master Electrical Component List in Wiring Systems.
1 | Did you perform the Diagnostic System Check for the system exhibiting the symptom? | Go to Step 2 | Go to Control Module References in Body Control System for the applicable Diagnostic System Check
2 |

1. Turn the ignition OFF.
2. Disconnect the body control module (BCM), engine control module (ECM), and transmission control module (TCM) connectors.
3. Test both the controller area network (CAN) high and low circuits for the following conditions:

• | A short to ground

• | A short to voltage

• | A short to each other

Turn the ignition ON, with the engine OFF when you test for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 5

Go to Step 3

3 |

1. Using the scan tool, clear the DTCs.
2. Turn the ignition OFF.
3. Reconnect each module one at a time, starting with the TCM, then the BCM, and finally the ECM.
4. After each module is connected, check for U2100 in the ECM.

Does DTC U2100 reset after reconnecting each module one at a time?

Go to Step 5

Go to Step 4

4 | Important: Perform the programming or setup procedure for the replaced control module if required. Replace the module that resets U2100. Refer to Control Module References in Body Control System for the applicable replacement procedure. Did you complete the replacement? | Go to Step 5 | --
5 |

1. Reconnect all modules not already connected.
2. Use the scan tool in order to clear the DTCs.
3. Turn the ignition switch OFF.
4. Wait at least 10 seconds.
5. Turn the ignition ON, with the engine OFF.
6. Retrieve DTCs from the ECM.

Does the DTC reset ?

Go to Step 2

System OK