Data Communications (Powertrain Expansion Bus without HP6)
DTC U0074 (Powertrain Expansion Bus without HP6)
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC U0074 00
- Control Module Communication Powertrain Expansion CAN Bus Off Malfunction
Diagnostic Fault Information
Circuit/System Description
The devices connected to the powertrain high speed CAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices when the ignition switch is in any position other than OFF. The powertrain high speed CAN serial data bus uses terminating resistors that are in parallel with the powertrain high speed CAN (+) and (-) circuits.
Conditions for Running the DTC
* The system voltage is between 9-16 V.
* The vehicle power mode requires serial data communications.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter device availability has not been received for more than 450 ms.
Action Taken When the DTC Sets
DTC U0074 is a type B DTC.
Conditions for Clearing the DTC
DTC U0074 is a type B DTC.
Diagnostic Aids
* Use the Data Link References Initial Inspection and Diagnostic Overview to identify the powertrain high speed CAN devices.
* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of-communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
- A device on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more devices was interrupted during diagnosis.
- A low battery condition was present, so some devices stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and devices on the communication circuit did not all re-initialize at the same time.
- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
* Do not replace a device reporting a U code. The U code identifies which device needs to be diagnosed for a communication issue.
* Communication may be available between some devices and the scan tool with the powertrain high speed CAN serial data system inoperative. This condition is due to those devices using multiple serial data communication systems.
* An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
* Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC).
* Some devices may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no device output voltage circuit is shorted to ground before replacing the device.
Reference Information
Schematic Reference
* Data Communication Schematics [1][2]Electrical Diagrams
* Control Module References Control Module References
Connector End View Reference
Component Connector End Views Sunshade - Left
Description and Operation
Data Link Communications Description and Operation Description and Operation
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References Control Module References for scan tool information
Circuit/System Testing
Note: Each device may need to be disconnected to isolate a circuit fault.
Use the schematic to identify the following:
* Powertrain high speed CAN devices the vehicle is equipped with
* Powertrain high speed CAN serial data circuit terminating resistors
* Device locations on the powertrain high speed CAN serial data circuits
* Each device's ground, B+, ignition, and powertrain high speed CAN serial data circuit terminals
Some devices with an internal terminating resistor have a loop in the harness that connects the internal terminating resistor to the serial data circuit. When wired this way, test these loop circuits for the appropriate failure mode short to voltage, short to ground, or open/high resistance prior to replacing the device for each of the following tests.
1. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. Disconnect the harness connectors with the powertrain high speed CAN serial data circuits at an easily accessible device that is not communicating. It may take up to 2 minutes for all vehicle systems to power down. Refer to Data Link References Initial Inspection and Diagnostic Overview to determine which devices are on the powertrain expansion bus.
2. Ignition ON.
3. Test for less than 4.5 V between each powertrain high speed CAN serial data circuit at the device that was just disconnected and ground.
• If 4.5 V or greater
Refer to Testing the Serial Data Circuits for a Short to Voltage.
• If less than 4.5 V
4. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. It may take up to 2 minutes for all vehicle systems to power down.
5. Test for greater than 100 ohms between each powertrain high speed CAN serial data circuit at the device that was just disconnected and ground.
• If 100 ohms or less
Refer to Testing the Serial Data Circuits for a Short to Ground.
• If greater than 100 ohms
6. Test for 110-130 ohms between each pair of the powertrain high speed CAN serial data circuits at the device that was just disconnected:
• If less than 110 ohms
Refer to Testing the Serial Data Circuits for a Short between the Circuits.
• If greater than 130 ohms
Refer to Testing the Serial Data Circuits for an Open/High Resistance.
• If between 110-130 ohms
7. Refer to Testing the Device Circuits.
Testing the Serial Data Circuits for a Short to Voltage
1. Ignition OFF, disconnect the harness connectors with the powertrain high speed CAN serial data circuits at another device, in the direction of the circuit shorted to voltage, ignition ON.
2. Test for greater than 4.5 V between each serial data circuit at the device connector that was just disconnected and ground.
• If each serial data circuit is 4.5 V or less
1. Ignition OFF.
2. Test for less than 10 ohms between each of the device's ground circuit terminals and ground.
• If 10 ohms or greater, repair the open/high resistance in the circuit.
• If less than 10 ohms, replace the device that was disconnected.
• If any serial data circuit is greater than 4.5 V
3. Repeat step 1 until one of the following conditions are isolated:
* A short to voltage on the serial data circuit between two devices or splice packs, if equipped.
* A short to voltage on the serial data circuit between a device and a terminating resistor.
Testing the Serial Data Circuits for a Short to Ground
1. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. It may take up to 2 minutes for all vehicle systems to power down.
2. Disconnect the harness connectors with the powertrain high speed CAN serial data circuits at another device, in the direction of the circuit shorted to ground.
3. Test for greater than 100 ohms between each serial data circuit at the device connector that was just disconnected and ground.
• If each serial data circuit is 100 ohms or greater
Replace the device that was disconnected.
• If any serial data circuit is less than 100 ohms
4. Repeat step 1 until one of the following conditions are isolated:
* A short to ground on the serial data circuit between two devices or splice packs, if equipped.
* A short to ground on the serial data circuit between a device and a terminating resistor.
Testing the Serial Data Circuits for a Short between the Circuits
1. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. It may take up to 2 minutes for all vehicle systems to power down.
2. Connect the harness connectors at the device that was previously disconnected.
3. Disconnect the harness connectors with the powertrain high speed CAN serial data circuits at another device, in the direction of the circuit shorted together.
4. Test for greater than 110 ohms between each pair of serial data circuits at the device connector that was just disconnected.
• If each pair of serial data circuits is 110 ohms or greater
Replace the device that was disconnected.
• If any pair of serial data circuits is less than 110 ohms
5. Repeat step 1 until one of the following conditions are isolated:
* Serial data circuits shorted together between two devices or splice packs, if equipped.
* Serial data circuits shorted together between a device and a terminating resistor.
* A shorted terminating resistor.
Testing the Serial Data Circuits for an Open/High Resistance
1. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. It may take up to 2 minutes for all vehicle systems to power down.
2. Connect the harness connectors at the device that was previously disconnected.
3. Disconnect the harness connectors with the powertrain high speed CAN serial data circuits at another device, in the direction of the circuit with the open/high resistance.
4. Test for less than 130 ohms between each pair of the serial data circuits at the device connector that was just disconnected.
• If each pair of serial data circuits is 130 ohms or less
Replace the device that was disconnected.
• If any pair of serial data circuits is greater than 130 ohms
5. Repeat step 1 until one of the following conditions are isolated:
* An open/high resistance on the serial data circuit between two devices or splice packs, if equipped.
* An open/high resistance on the serial data circuit between a device and a terminating resistor.
* An open/high resistance terminating resistor.
Testing the Device Circuits
1. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. It may take up to 2 minutes for all vehicle systems to power down.
2. Test for less than 10 ohms between each ground circuit terminal at the device that was just disconnected and ground.
• If 10 ohms or greater
1. Ignition OFF.
2. Test for less than 2 ohms in the ground circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, repair the open/high resistance in the ground connection.
• If less than 10 ohms
3. If equipped, verify a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition OFF.
2. Test for less than 2 ohms in the B+ circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is not open and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition OFF.
2. Test for infinite resistance between the B+ circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the disconnected device.
• If the test lamp illuminates
4. Ignition ON.
5. If equipped, verify a test lamp illuminates between each ignition circuit terminal, which has a fuse in the circuit, and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition OFF.
2. Test for less than 2 ohms in the ignition circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is OK and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition OFF.
2. Test for infinite resistance between the ignition circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the disconnected device.
• If the test lamp illuminates
6. If equipped, verify a test lamp illuminates between each ignition circuit terminal, which is controlled by a control module, and ground.
• If the test lamp does not illuminate
1. Ignition OFF, disconnect the harness connectors at the control module that controls the ignition circuit.
2. Test for infinite resistance between the ignition circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 ohms in the ignition circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the control module that controls the ignition circuit.
• If the test lamp illuminates
7. Replace the device that was disconnected.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the repair.
* GMLAN and Media Oriented Systems Transport (MOST) Wiring Repairs Service and Repair
* Control Module References Control Module References for device replacement, programming and setup