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 U0020: Low Speed CAN Communication Bus Performance

### Circuit/System Description

Modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have programmed information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module.

### Conditions for Running the DTC

• | Voltage supplied to the modules is in the normal operating voltage range.

• | The vehicle power mode requires serial data communication to occur.

• | DTC U2100 does not have a current status.

### Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received.

### Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

### Reference Information

#### Schematic Reference

• | Data Communication Schematics

• | Body Control System Schematics

#### Connector End View Reference

Data Communication Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Output Controls

• | Scan Tool Data List

• | Scan Tool Data Definitions

### Circuit/System Testing

1. With a scan tool, verify communication with one or more modules is present.

⇒ | If no communication is available refer to Scan Tool Does Not Communicate with Low Speed GMLAN Device .

2. Ignition OFF, disconnect the C4 harness connectors at the BCM.
3. Ignition OFF, test for less than 1 ohm of resistance between the ground circuit terminal E 12 and ground.

⇒ | If greater than the specified value, test the circuit for an open/high resistance.

4. Ignition OFF, disconnect the C1 harness connectors at the BCM.
5. Ignition OFF, test for less than 1 ohm of resistance between the ground circuit terminal 61 and ground.

⇒ | If greater than the specified value, test the circuit for an open/high resistance.

6. Reconnect the C1 and C4 harness connector and disconnect the C3 harness connector at the BCM.
7. Ignition ON, verify a test lamp illuminates between the B+ circuit terminal D1 and ground.

⇒ | If test lamp does not Illuminate, test the circuit for an open/high resistance. If the circuit fuse is open, test the circuit for a short to ground. If all circuits test normal, replace the BCM.

8. Ignition OFF, disconnect the harness connector of the non-communicating module.
9. Ignition OFF, test for less than 1 ohm of resistance between the applicable modules ground circuit terminal and ground.

⇒ | If greater than the specified value, test the circuit for an open/high resistance.

10. Ignition ON, verify a test lamp illuminates between the applicable modules B+ circuit and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance.

11. Individually disconnect the harness connector(s) at each control module on the GMLAN serial data line, beginning with the module furthest from the DLC, as noted on the system schematic. After each disconnection, verify the scan tool is unable to communicate with the BCM.

⇒ | If the scan tool is able to communicate with the BCM, test the GMLAN serial data line at the most recently disconnected module.

12. If all circuits test normal, replace the applicable module.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Refer to Control Module References for control module replacement, setup, and programming.

• | Refer to GMLAN Wiring Repairs for repair instructions.