• Perform the Diagnostic System Check prior to using this diagnostic procedure:Diagnostic System Check - Vehicle
• Review the description of Strategy Based Diagnosis:Strategy Based Diagnosis
• An overview of each diagnostic category can be found here:Diagnostic Procedure Instructions

Control modules connected to the serial data circuits monitor the communication during normal vehicle operation. Operating information and commands are exchanged among the control modules. Each module on the serial data circuit maintains a transmit error counter and a receive error counter. The counter values increase with detected errors and decrease with error-free messages.

• DTC U0121 = Not set
• Ignition=On
• Serial Data=Enabled

Frequency the DTC runs=Continuously—After the running conditions are met—For greater than 3 s

Serial Data=Message Counter Incorrect—Between the following control modules:

• K17 Electronic Brake Control Module
• K71 Transmission Control Module

The above condition(s) must occur 5 times.

DTCs listed in the DTC Descriptor category=Type C

DTCs listed in the DTC Descriptor category=Type C

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Control Module References

1. Ignition » On / Vehicle » In Service Mode
2. Verify there are no DTCs set related to the following component/system:Serial Data
• If other DTCs are set

  Refer to:Diagnostic Trouble Code (DTC) List - Vehicle

• If no other DTCs are set
1. Ignition/Vehicle » Off
2. Replace the component:K17 Electronic Brake Control Module
3. Operate the vehicle within the Conditions for Running the DTC.

   Verify the DTC does not set.

• If the DTC sets » Replace the component:K71 Transmission Control Module
• If the DTC is not set
4. All OK.
• If no DTC is set
3. All OK.

Perform the Diagnostic Repair Verification after completing the repair:Diagnostic Repair Verification

For control module replacement, programming, and setup refer to:Control Module References