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Chevy U Codes

This bulletin applies to all Chevrolet vehicles model years 2013-2018. When diagnosing multiple DTCs, one may be 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. A symptom byte 71 indicates invalid data. This sub type is used by the ECU to indicate a signal was received but invalid.

In these cases, the DTC list should be examined for what control module or missing data the U codes point to and check for DTCs related to the managing control module’s 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.

An example is an intermittent wheel speed sensor code is setting U codes across buses and the root cause was tracked back to the wheel speed sensor and EBCM messages. 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 Stabilitrak” and “Service ABS” messages. This is in addition to several U codes corresponding to these messages which may include U0121, U0131, B1011, U0414, C0040, U0422, U0415, U2502, U0401, U0403 and U0402.

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

The multiple U codes can cause misdiagnosis. An erratic wheel speed signal can cause this failure due to miscommunication in the system when it intermittently fails. When you review 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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