DTC U0020
Diagnostic Instructions
DTC Descriptor
DTC U0020 00: Low Speed CAN Communication Bus Performance
Circuit/System Description
Control modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control modules 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 module as an availability indication of the transmitter module.
DTC U0020 will set when fewer control modules are detected on the bus before the control modules can learn the other control modules IDs. If the communication bus is opened, or if the control modules power supply is lost when the vehicle is in the OFF power mode, and then placed in the ON power mode, U0020 will set in specific control modules.
Conditions for Running the DTCs
| • | Voltage at the control modules is in the normal operating voltage 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 module availability has not been received.
Action Taken When the DTCs Sets
The control module 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 control module 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 Symptoms - Data Communications and DTC U0100-U02FF .
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