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 circuit about once every two seconds. When the body control module (BCM) and radio receiver detects no messages and a high voltage on the Class 2 serial data circuit for approximately three seconds, U1301 is set and the setting of all other communication DTCs is inhibited.

### Conditions for Setting the DTC

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

• | 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 all met for approximately three seconds.

### Conditions for Clearing the DTC

• | A current 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 a current U1301 is accomplished via the symptom, Scan Tool Will Not Communicate With Class 2 Data Line.

• | An intermittent condition is likely to be caused by a short to voltage on the Class 2 serial data circuit. Use the Scan Tool Will Not Communicate With Class 2 Data Line procedure to isolate the condition.