U0074
DTC U0074
Diagnostic Instructions
* Perform theDiagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* ReviewStrategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.
DTC Descriptor
DTC U0074 00
- Control Module Communication Bus B Off
Diagnostic Fault Information
Circuit/System Description
The PTLAN serial data circuits are serial data buses used to communicate information between the control modules. The PTLAN serial data circuits also connect directly to the data link connector (DLC) for programming purposes. If this serial data system is inoperative the modules will still communicate with the scan tool due to other serial data systems but not with each other. Communication with the scan tool for the modules on the PTLAN bus is through the serial data gateway module (SDGM) on the HSGMLAN serial data system.
Conditions for Running the DTCs
* Supply voltage to the modules are in the normal operating range.
* The ignition switch in any position other than START.
Conditions for Setting the DTC
The module setting the DTC has attempted to establish communications on the PTLAN serial data circuits more than 3 times.
Action Taken When the DTCs Sets
* The module suspends all message transmission.
* The MIL and battery indicators will illuminate, along with several DIC messages.
* The vehicle may continue to run for several minutes, then will stall and not start.
Conditions for Clearing the DTC
* A current DTC clears when the malfunction is no longer present.
* A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.
Diagnostic Aids
* When testing circuits to the SDGM access can be accomplished by removing the instrument panel center trim panel. Note that this procedure does not allow removal of the SDGM, only access to the connector. Refer to Instrument Panel Center Trim Panel Replacement (with RPO SLT) (Service and Repair) .
* Circuits to the HPCM are diagnosed only to the PIM connector because the HPCM is replaced as part of the PIM.
* Different DTCs may set in different modules along with this DTC depending on where the fault occurs and the vehicle operating conditions when the fault occurs.
Reference Information
Schematic Reference
* Data Communication Schematics (Electrical Diagrams)
* Control Module References (Programming and Relearning)
Connector End View Reference
Component Connector End Views (Connector Views)
Description and Operation
Data Link Communications Description and Operation (Description and Operation)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
Circuit/System Testing
Initial Test
1. Ignition ON, verify 4-6 volts between the PTLAN circuits of the DLC connector terminals 3 and 11.
• If not within the specified range, refer to Active U0074 Test.
2. Verify that DTC U18xx is not set in any module listed.
* ECM
* HPCM
* SDGM
• If DTC U18xx is set in any of the above modules, refer toDiagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code Descriptions) .
Active U0074 Test
1. With the ignition OFF, disconnect the module/component harness connectors listed below one at a time. After each disconnect and with the ignition ON, test for 4-6 volts between PTLAN circuits of DLC connector terminals 3 and 11 until the voltage is within specified range.
1. Data link resistor 2 - X1
2. Yaw rate and lateral acceleration sensor - X1
3. Engine control module (ECM) - X3
4. Hybrid powertrain control module (HPCM) - X1
5. Accessory DC power control module (APM) - X1
6. Electronic brake control module (EBCM) - X1
7. Serial data gateway module (SDGM) - X1
• If all module/components are disconnected and not within the specified range, test all 4 PTLAN circuits of SDGM connector X1 for a short between them, short to voltage, or a short to ground. If the circuits test normal, replace the SDGM.
Important: If either the data link resistor or the ECM is the module/component disconnected before voltage at the DLC terminals is within the specified range no further testing is necessary, replace the last module/component disconnected.
2. Test for infinite resistance between both PTLAN circuits of the last module disconnected that lead to the previously disconnected module/component.
• If less than the specified value, test the circuits for a short together.
3. Test for infinite resistance between ground and both PTLAN circuits of the last module disconnected that lead to the previously disconnected module/component.
• If less than the specified value, test the circuit for a short to ground.
4. Test for less than 1 volt between ground and both PTLAN circuits of the last module disconnected that lead to the previously disconnected module/component.
• If greater than the specified range, test the circuit for a short to voltage.
5. If the circuits test normal, replace the last module disconnected.
Repair Instructions
Perform theDiagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* GMLAN Wiring Repairs (Component Tests and General Diagnostics)
* ABS Description and Operation (Description and Operation) for the ABS - Hybrid
* Vehicle Yaw Sensor with Vehicle Lateral Accelerometer Replacement (Less than 3900 kg (8600 lb) GVW) for the ABS - Non-Hybrid Less than 3900 kg (8600 lb) GVW
* Vehicle Yaw Sensor with Vehicle Lateral Accelerometer Replacement (Less than 3900 kg (8600 lb) GVW) for the ABS - Non-Hybrid Greater than/Equal to 3900 kg (8600 lb) GVW
* Control Module References (Programming and Relearning) for module replacement, setup, and programming