Circuit Description
The link for serial data communications during normal vehicle operation
is monitored by modules that are connected to the Class 2 serial data
link. Operating information and commands are exchanged among the modules.
The Node Alive (State of Health) messages are transmitted by each module
on the Class 2 serial data link about once every 2 seconds.
The DTC U1255 is set when the module does not detect a message on the Class 2
serial data link that the module expects to receive.
Conditions for Setting the DTC
| • | The voltage is supplied to the module in the normal operating
voltage range of approximately 9-16 Volts. |
| • | A Class 2 message that the module expects to receive is
not detected on the data link. |
Conditions for Clearing the DTC
| • | When a valid Class 2 message is detected on the Class 2
serial data link or at the end of the current ignition cycle, a current U1255
will clear. |
| • | A history U1255 will clear upon receipt of a scan tool CLEAR DTCs
command. |
Diagnostic Aids
| • | Diagnosis of a current U1255 is accomplished via the symptom.
The scan tool will not communicate with a Class 2 device. |
| • | An intermittent condition is likely to be caused by a short to
ground on the Class 2 serial data link. |
| • | When this DTC is set by the BCM, the engine may not run. When
this DTC is set by the BCM and the engine runs, an ignition feed to either
the BCM or the VCM/PCM is open. |
