For 1990-2009 cars

### Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. In addition to this, Node Alive (State of Health) messages are transmitted by each module on the class 2 serial data circuit about once every two seconds. When the module detects no messages and a high voltage on the Class 2 serial data link for approximately three seconds, U1301 is set and the setting of all other class 2 serial communication DTCs is inhibited.

### Conditions for Running the DTC

• | Voltage supplied to the module is in the normal operating voltage range (approximately 9 - 16 volts).

• | The vehicle power mode (ignition switch position) requires serial data communication to occur.

### Conditions for Setting the DTC

• | No valid class 2 messages are detected on the class 2 serial data circuit.

• | The voltage level detected on the class 2 serial data circuit is high.

• | The above conditions are met for approximately three seconds.

### Action Taken When the DTC Sets

• | The module inhibits the setting of all other class 2 DTCs.

• | The module uses default values for all parameters received on the class 2 serial data circuit.

### Conditions for Clearing the DTC

• | A U1301 will clear when a valid class 2 message is detected on the class 2 serial data circuit or at the end of the current ignition cycle.

• | A history U1301 will clear upon receipt of a scan tool Clear DTCs command.

### Diagnostic Aids

• | This DTC cannot be retrieved with a current status. Diagnosis of an U1301 is accomplished via the symptom, Scan Tool Does Not Communicate with a Class 2 Device.

• | An intermittent condition is likely to be caused by a short to voltage on the class 2 serial data circuit. Refer to Intermittents and Poor Connections Diagnosis . to isolate an intermittent condition.