U0020

DTC U0020

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.

* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC U0020 00
- Low Speed CAN Communication Bus Performance Malfunction

Circuit/System Description

Devices connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices. The devices have programmed information about what messages are needed to be exchanged on the serial data circuits. The messages are also supervised and some periodic messages are used by the receiver device as an availability indication of the transmitter device.

DTC U0020 will set when fewer devices are detected on the bus before the devices can learn the other device IDs. If the communication bus is opened, or if the device power supply is lost when the vehicle is in the OFF power mode, and then placed in the RUN power mode, U0020 will set in specific devices.

Conditions for Running the DTCs

* Supply voltage to the devices is in the normal operating range.

* The vehicle power mode requires serial data communication to occur.

* The DTC U2100 does not have a current status.

Conditions for Setting the DTC

A supervised periodic message that includes the transmitter device availability has not been received.

Action Taken When the DTCs Sets

The device uses a default value for the missing parameter.

Conditions for Clearing the DTC

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

* A history DTC clears when the device ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

Circuit/System Verification

Diagnosis of this DTC is accomplished via the symptom or an additional DTC. Refer to Scan Tool Does Not Communicate with Low Speed GMLAN Device Scan Tool Does Not Communicate with Low Speed GMLAN Device or Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle.