Technical Service Bulletins

U-Code Misdiagnosis

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U-Code Misdiagnosis

This bulletin applies to all Chevy Impala vehicles built from 2013-2018, when diagnosing multiple DTCs, one of which includes U****Byte 71. Any control module may set a DTC for one of its inputs, and that data is shared across the communication bus for other modules to perform their respective functions.

The receiving module may set a 71 byte type DTC (symptom byte 71; symptom byte description: invalid data). This sub type is used by the ECM to indicate a signal was received with the corresponding validity bit equal to invalid or post processing of the signal it determines is invalid.

In some cases the DTC list should be examined for which control module or missing data the U code points to and check for DTCs related to the managing control modules’ inputs.

As a general rule, never replace components for “invalid data received” codes within the component. Resolve the system-wide non-communication (not U codes) first and re-evaluate.

The DIC may show “service stabilitrack” and “service ABS.” When the EBCM sets any of the wheel sensor codes C0035-C0050, it yields "Service Transmission,” “Steering Assist Reduced” and “AWD off” in addition to “Service Stabilitrack” and “Service ABS” messages.

As an example, the EBCM would set the wheel speed sensor code intermittently, triggering a system malfunction message to be transmitted, resulting in multiple U codes across the chassis expansion and HS CAN buses. The multiple U codes can cause misdiagnosis.

An erratic wheel speed signal can cause this failure due to the miscommunication in the system when it intermittently fails. When you review the ABS data, you may find an erratic signal from the wheel speed sensor. This can be caused by a faulty front hub wheel bearing.

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