For 1990-2009 cars

Class II serial data is a 0-7 volt variable pulse width message scheme, logic 0's and logic 1's, used to communicate information between modules and for communication to an off-board scan tool. Contained in each module memory is a list of serial data messages and default actions taken by the module if one of the important messages is not received. One of the most important messages is the state of health (SOH) message used to inform each module that other modules on the Class II link are functioning correctly.

Each module on the Class II link must first "learn" all modules connected to the Class II link every time the ignition is turned ON. Each message that a module sends out on the link contains a source identifier used to identify where the information came from. Once a module receives information, it will store the attached controller ID. If this same information is not received later in the ignition cycle, lost communications with a specific controller DTC will set. DTC U1000 sets when a controller does not receive specific information when the source identifier was not learned.

### DTC Parameters

DTC U1000 will set if the BCM, PCM, or SDM did not receive information from another Class II module when:

• | The condition exists for longer than 6 seconds.

• | The BCM, PCM or SDM was not able to "learn" the source identifier of another Class II module that it relies on for information.

• | The ignition is ON.

Possible combinations for U1000:

#### 4 Cylinder L61

• | The PCM cannot communicate with the BCM or SDM.

• | The BCM cannot communicate with the PCM or SDM.

• | The SDM cannot communicate with the BCM, I/P cluster, or PCM.

#### 6 Cylinder L81

• | The BCM can not communicate with the SDM.

• | The SDM can not communicate with the BCM, ECM on CAN link, or the I/P cluster.

The DTC U1000 diagnostic runs continuously with the ignition ON.

DTC U1000 is a type C DTC.

### Diagnostic Aids

Important: If only one other U-type DTC is set, diagnose that DTC first. If 2 or more other U-type DTCs are set, perform the "Vehicle On-Board Diagnostic System Check" located in this section.

This DTC indicates that a module did not communicate with another module when the ignition was first turned ON.

The Class II circuit is in a ring configuration. An individual open in circuit 1807 will NOT affect communications.

Check for module connector tightness and broken connector locks.

To locate an intermittent problem, use the scan tool to monitor each individual controller located under DIAGNOSTIC CIRCUIT CHECK - CLASS II MESSAGE MONITOR with the ignition ON and the engine OFF. INACTIVE will be displayed if a controller is not communicating on the Class II link.