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 U0073: CAN-Bus Circuits Malfunctions

### Circuit Description

GMLAN serial data circuit is CAN serial data bus, which is used to communicate information between each control module. Keep the normal data transmission speed high enough to ensure real time response as required. This vehicle is equipped with two different GMLAN serial data circuits, those are High Speed Dual-Circuited and Low Speed Single-Circuited. GMLAN serial data circuits are also communicating with Data Link Connector (DLC) directly. The information is interpreted by CANdi module which is connected outside and is equal to a scan tool receiver.

Each module connected to the GMLAN serial data circuit monitors the communication during normal vehicle operation, operating information is exchanged among modules during this procedure. Each module on GMLAN network supports 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.

### Conditions for Running the DTCs

• | Each module is within the normal operating supply voltage range.

• | The vehicle is in the RUN power mode.

### Conditions for Setting the DTC

The module that sets DTC attempts to establish communication on GMLAN circuit more than 5 times or 5 seconds

### Action Taken When the DTCs Sets

• | The module suspends all message transmission.

• | The module uses default values for all parameters received on the serial data circuits.

• | The module inhibits the setting of all other communication DTCs.

### Conditions for Clearing the DTC

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

• | When the ignition cycle counter has reached 50 and no trouble occurs repeatedly, the history DTC will be cleared.

### Diagnostic Aids

• | This DTC cannot be retrieved with a current status. Based on module serial data communication, diagnosis of current DTC is accomplished via the symptoms, Scan Tool Does Not Communicate with High Speed GMLAN device or Scan Tool Does Not Communicate with Low Speed GMLAN device.

• | Refer to Data Link References for the specific information regarding modules using GMLAN serial data.