DTC U0076

Diagnostic Instructions Perform the Diagnostic System Check prior to using this diagnostic procedure: Diagnostic System Check - Vehicle Review the description of Strategy Based Diagnosis: Strategy Based Diagnosis An overview of each diagnostic category can be found here: Diagnostic Procedure Instructions DTC Descriptor Control Module Communication Powertrain Sensor CAN Bus Off For symptom byte information, refer to Symptom Byte List. Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Powertrain Sensor Bus Enable	U100B	U0100– U02FF, U1100– U12FF, U1600– U17FF, U2200– U23FF, U2400– U25FF*	U100B	—
Powertrain CAN Serial Data [+] (circuit 4055)	U0076*	U0100– U02FF, U1100– U12FF, U1600– U17FF, U2200– U23FF, U2400– U25FF*	U0076*	—
Powertrain CAN Serial Data [−] (circuit 4054)	U0076*	U0100– U02FF, U1100– U12FF, U1600– U17FF, U2200– U23FF, U2400– U25FF*	U0076*	—
Other DTCs may set with this fault.

Circuit/System Description The devices connected to the Powertrain CAN serial data circuits (4054 & 4055) monitor for serial data communications during normal vehicle operation. The serial data is transmitted on two twisted wires that allow speeds up to 2 Mbit/s. The twisted pair is terminated with two 120 Ω resistors, one is internal to the K20 Engine Control Module (ECM) and the other can be a separate resistor in a connector assembly or in another device. The resistors are used as the load for the Powertrain CAN bus during normal vehicle operation. The Powertrain CAN bus is a differential bus. The Powertrain CAN serial data (+) and Powertrain CAN serial data (−) circuits are driven to opposite extremes from a rest or idle level of approximately 2.5 V. Driving the lines to their extremes, adds 1 V to the Powertrain CAN serial data (+) circuit and subtracts 1 V from the Powertrain CAN serial data (−) circuit. If serial data is lost, devices will set a no communication code against the non-communicating device. Note that a loss of serial data DTC does not represent a failure of the device that set it. 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 The device setting the DTC has attempted to establish communications on the Powertrain CAN serial data circuits more than 3 times. Action Taken When the DTC Sets The device suspends all message transmission. The device uses default values for all parameters received on the serial data circuits. The device inhibits the setting of all other communication DTCs. Conditions for Clearing the DTC The K20 Engine Control Module or K71 Transmission Control Module turns OFF the MIL after 4 consecutive ignition cycles that the diagnostic runs and does not fail. A current DTC will clear when the diagnostic runs and passes. A history DTC will clear after 100 consecutive malfunction-free ignition cycles. Diagnostic Aids 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 CAN serial data system inoperative. This condition is due to those devices using multiple serial data communication systems. The engine may not start when there is a total malfunction of a Powertrain CAN serial data bus. 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 Control Module References Connector End View Reference Master Electrical Component List Description and Operation Data Link Communications Description and Operation Electrical Information Reference Circuit Testing Connector Repairs Testing for Intermittent Conditions and Poor Connections Wiring Repairs Scan Tool Reference Control Module References for scan tool information Circuit/System Verification

1. Ignition ON/Vehicle In Service Mode.
2. Verify that the scan tool is communicating with vehicle. ⇒ If the scan tool is not communicating with vehicle Refer to Scan Tool Does Not Communicate with Vehicle. ⇓ If the scan tool is communicating with vehicle
3. Attempt to communicate with all devices on the Powertrain CAN bus. Refer to Data Link References to determine how many devices should be communicating on the Powertrain CAN bus.
4. Verify two or more devices are not communicating on the Powertrain CAN bus. ⇒ If only one device is not communicating Refer to Circuit/System Testing – Testing the Device Circuits. ⇓ If two or more devices are not communicating
5. Refer to Circuit/System Testing – Serial Data Starting Point. Circuit/System Testing

Note

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. Each device may need to be disconnected to isolate a circuit fault. Use the schematics to identify the following: Powertrain CAN devices and terminating resistors the vehicle is equipped with Device locations on the Powertrain CAN serial data circuits Each device’s ground, B+, ignition, and Powertrain CAN serial data circuit terminals Serial Data Starting Point

1. Ignition/Vehicle OFF, disconnect the harness connectors with the Powertrain CAN serial data circuits at an easily accessible device that is not communicating.
2. Ignition ON/Vehicle In Service Mode.
3. Verify a test lamp illuminates between the powertrain sensor bus enable circuit terminal and ground. ⇒ If the test lamp does not illuminate 3.1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K20 Engine Control Module and all devices that share the powertrain sensor bus enable circuit. 3.2. Test for infinite resistance between the powertrain sensor bus enable circuit and ground. ⇒ If less than infinite resistance, repair the short to ground on the circuit. ⇓ If infinite resistance 3.3. Test for less than 2 Ω in the powertrain sensor bus enable circuit end to end. ⇒ If 2 Ω or greater, repair the open/high resistance in the circuit. ⇓ If less than 2 Ω 3.4. Reconnect the K20 Engine Control Module, ignition ON/Vehicle In Service Mode. 3.5. Verify a test lamp illuminates between the powertrain sensor bus enable circuit terminal and ground. ⇒ If the test lamp does not illuminate, replace the K20 Engine Control Module. ⇓ If the test lamp illuminates 3.6. Ignition/Vehicle OFF, reconnect the first device that shares the powertrain sensor bus enable circuit, ignition ON/Vehicle In Service Mode. 3.7. Verify a test lamp illuminates between the powertrain sensor bus enable circuit terminal and ground. ⇒ If the test lamp does not illuminate, replace the device that was just connected. ⇒ If the test lamp illuminates and there are more devices to connect, connect the next device and repeat step 3.7 until there are no more devices to connect. ⇓ If the test lamp illuminates
4. Test for less than 4.5 V between each of the Powertrain CAN serial data circuit terminals 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
5. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
6. Test for greater than 100 Ω between each of the Powertrain CAN serial data circuit terminals at the device that was just disconnected and ground. ⇒ If 100 Ω or less Refer to Testing the Serial Data Circuits for a Short to Ground. ⇓ If greater than 100 Ω
7. Test for 110–130 Ω between each pair of the Powertrain CAN serial data circuit terminals at the device that was just disconnected. ⇒ If less than 110 Ω Refer to Testing the Serial Data Circuits for a Short between the Circuits. ⇒ If greater than 130 Ω Refer to Testing the Serial Data Circuits for an Open/High Resistance. ⇓ If between 110–130 Ω
8. Replace the device that was just disconnected. Testing the Serial Data Circuits for a Short to Voltage
9. Ignition/Vehicle OFF, disconnect the harness connectors with the Powertrain CAN serial data circuits at another device, in the direction of the circuit shorted to voltage.
10. Ignition ON/Vehicle In Service Mode
11. Test for greater than 4.5 V between each of the Powertrain CAN serial data circuits at the device connector that was just disconnected and ground. ⇒ If each serial data circuit is 4.5 V or less Replace the device that was just disconnected. ⇓ If any serial data circuit is greater than 4.5 V
12. 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
13. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
14. Disconnect the harness connectors with the Powertrain CAN serial data circuits at another device, in the direction of the circuit shorted to ground.
15. Test for greater than 100 Ω between each of the Powertrain CAN serial data circuits at the device connector that was just disconnected and ground. ⇒ If each serial data circuit is 100 Ω or greater Replace the device that was just disconnected. ⇓ If any serial data circuit is less than 100 Ω
16. 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
17. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
18. Connect the harness connectors at the device that was previously disconnected.
19. Disconnect the harness connectors with the Powertrain CAN serial data circuits at another device, in the direction of the circuit shorted together.
20. Test for greater than 110 Ω between each pair of the Powertrain CAN serial data circuits at the device connector that was just disconnected. ⇒ If each pair of serial data circuits is 110 Ω or greater Replace the device that was just disconnected. ⇓ If any pair of serial data circuits is less than 110 Ω
21. 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
22. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
23. Connect the harness connectors at the device that was previously disconnected.
24. Disconnect the harness connectors with the Powertrain CAN serial data circuits at another device, in the direction of the circuit with the open/high resistance.
25. Test for less than 130 Ω between each pair of the Powertrain CAN serial data circuits at the device connector that was just disconnected. ⇒ If each pair of serial data circuits is 130 Ω or less Replace the device that was just disconnected. ⇓ If any pair of serial data circuits is greater than 130 Ω
26. 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
27. Ignition/Vehicle OFF, disconnect the harness connectors at the Powertrain CAN device that is not communicating.
28. Verify a test lamp illuminates between each of the ground circuit terminals and B+. ⇒ If the test lamp does not illuminate Repair the open/high resistance in the ground circuit or ground connection. ⇓ If the test lamp illuminates
29. Ignition ON/Vehicle In Service Mode.
30. If equipped, verify a test lamp illuminates between each of the B+ circuit terminals and ground. ⇒ If the test lamp does not illuminate and the circuit fuse is good 4.1. Test for an open/high resistance in the B+ circuit. ⇒ If an open/high resistance is found, repair the fault on the circuit. ⇓ If an open/high resistance is not found 4.2. 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 4.1. Test for a short to ground in the B+ circuit. ⇒ If a short to ground is found, repair the fault in the circuit. ⇓ If a short to ground is not found 4.2. Replace the disconnected device. ⇓ If the test lamp illuminates
31. If equipped, verify a test lamp illuminates between each of the ignition circuit terminals, which has a fuse in the circuit, and ground. ⇒ If the test lamp does not illuminate and the circuit fuse is good 5.1. Test for an open/high resistance in the ignition circuit. ⇒ If an open/high resistance is found, repair the fault in the circuit. ⇓ If an open/high resistance is not found 5.2. 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 5.1. Test for a short to ground in the ignition circuit. ⇒ If a short to ground is found, repair the fault in the circuit. ⇓ If a short to ground is not found 5.2. Replace the disconnected device. ⇓ If the test lamp illuminates
32. If equipped, verify a test lamp illuminates between the powertrain sensor bus enable circuit terminal and ground. ⇒ If the test lamp does not illuminate 6.1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K20 Engine Control Module. 6.2. Test for less than 2 Ω in the powertrain sensor bus enable circuit end to end. ⇒ If 2 Ω or greater, repair the open/high resistance in the circuit. ⇒ If less than 2 Ω, replace the K20 Engine Control Module. ⇓ If the test lamp illuminates
33. Test for less than 4.5 V between each of the Powertrain CAN serial data circuit terminals 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
34. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
35. Test for greater than 100 Ω between each of the Powertrain CAN serial data circuit terminals and ground. ⇒ If 100 Ω or less Refer to Testing the Serial Data Circuits for a Short to Ground. ⇓ If greater than 100 Ω
36. Test for greater than 35 Ω between each pair of the Powertrain CAN serial data circuit terminals. ⇒ If 35 Ω or less Refer to Testing the Serial Data Circuits for a Short between the Circuits. ⇓ If greater than 35 Ω
37. Test for less than infinite resistance between each of the Powertrain CAN serial data circuit terminals and ground. ⇒ If infinite resistance Repair the open/high resistance in the serial data circuit. ⇓ If less than infinite resistance
38. Replace the non communicating device. Repair Instructions Perform the Diagnostic Repair Verification after completing the repair: Diagnostic Repair Verification. For control module replacement, programming, and setup refer to: Control Module References