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. When no communication on the CAN serial data line is detected, DTC U2104 will result.

### Conditions for Running the DTC

The CAN circuits are monitored continuously by the modules on the circuit. The only requirement is that the modules are in a RUN power mode.

### Conditions for Setting the DTC

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

### Action Taken When the DTC Sets

• | The DTC is set as both current and history, the modules on the CAN network each take separate actions as listed:

• | ECM and BCM:

- | Turn ON the malfunction indicator lamp (MIL) during the second consecutive drive cycle with the error detected.

- | Record the operating conditions at the time of turning ON the MIL and store the data as Freeze Frame information.

### Conditions for Clearing the MIL/DTC

• | A current DTC clears and the MIL is turned OFF when the fault is no longer detected after the 3rd consecutive drive cycle.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the fault, or does not reoccur after 40 consecutive drive cycles.

• | A history DTC clears if there is a battery disconnect.

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. Install the scan tool.
2. With the scan tool, communicate with the body control module (BCM), engine control module (ECM), and transmission control module (TCM).
3. Monitor the DTC information.

Does the scan tool indicate that DTC U2100 sets as either a current or history DTC?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3 | Important: Turn the ignition ON, with the engine OFF when testing for a short to voltage. Use a digital Multi Meter (DMM) MIN/MAX function to capture intermittent conditions. Test the CAN serial data circuits for the following:

• | A short to ground

• | A short to voltage

• | A short across CAN high and CAN low circuits

• | An open

Refer to the following in Wiring Systems:

• | Testing for Intermittent Conditions and Poor Connections

• | Circuit Testing

• | Connector Repairs

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4 | Inspect for poor connections at the harness connector of the module that is not communicating. Did you find and correct the condition? | Go to Step 6 | Go to Step 5
5 | Important: Perform the programming or setup procedure for the replaced control module if required. Replace the control module that is not communicating. Refer to Control Module References in Body Control System for the applicable replacement procedure.

Did you complete the replacement?

Go to Step 6

--

6 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition switch.
3. Wait at least 10 seconds.
4. Turn the ignition ON, with the engine OFF.
5. Retrieve the DTCs from the ECM, BCM, and TCM.

Does the DTC reset?

Go to Step 2

System OK