Diagnosing Communication U Codes before Replacing Parts

If the Driver Information Center (DIC) displays a “Steering Assist Reduced” message on a vehicle that comes into the dealership for service, it makes sense to check the steering components, right? But what if there are two DTCs set: P0128 (Engine Coolant Temperature Below Thermostat Regulating Temperature) and U0401:71 (Invalid Data Received From Engine Control Module)?


When diagnosing a vehicle with multiple Diagnostic Trouble Codes (DTC) set, it’s critical to understand each code and why it may have set. 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. (Fig. 6) The receiving module may set a network communication U code.


Fig. 6


Symptom Byte 71


DTCs with a symptom byte of 70-7F fall into the category of Bus Signal/Message Failures. This category includes faults related to bus hardware and signal integrity. It’s also used when the physical input for a signal is located in one control module and another control module diagnoses the circuit.


Symptom byte 71 is set when there is invalid data, which means the control module received a signal with the corresponding validity bit equal to invalid or post processing of the signal determines it’s invalid.


In these cases, the DTC list should be checked to determine which control module the missing data U code points to. Also look for DTCs that are related to the inputs of the managing control modules.


TIP: Never replace components for Invalid Data Received codes within the component. Resolve the system-wide non-communication first, not the U code.


U Code Examples


The “Steering Assist Reduced” DIC message mentioned earlier is a condition recently found on some 2017 XT5 and Acadia models. In some cases, the steering gear has been replaced without correcting the actual cause of the condition.


The “Steering Assist Reduced” message is displayed due to an invalid coolant temperature signal from the Engine Control Module (ECM). The Power Steering Control Module looks for a valid coolant temperature so it can compensate for cold and thick grease. As grease gets cold, it becomes thicker and harder to move, which can cause a heavier than normal steering feel. When the steering module thinks it’s cold due to the coolant temperature value, it provides extra assist to support the heavier condition. Since the coolant value on the CAN bus went invalid, this function of the steering module went inoperative.


For this repair, DTC P0128 should be diagnosed first, which is most likely related to a failing thermostat. Once the cause of that DTC is resolved, DTC U0401 can be cleared.


Another example covers several U codes corresponding to a number of DIC messages. An erratic wheel speed sensor signal caused the Electronic Brake Control Module (EBCM) to set wheel sensor codes C0035-C0050 and several U codes.


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. An erratic wheel speed signal can cause this failure due to the miscommunication in the system when it intermittently fails. Checking the ABS data on a scan tool would show an erratic signal from the wheel speed sensor. A faulty front hub wheel bearing was the cause of the erratic wheel speed sensor signal.


In general, when diagnosing multiple DTCs that include network communication codes, think about what DTCs could be caused by another DTC.


– Thanks to Bob Wittmann and Len Tillard


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