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U2401




DTC U2401

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 U2401 00
- Battery Energy Control Module Dedicated Bus 1 Off

Diagnostic Fault Information






Circuit/System Description

The battery energy control module (BECM) dedicated bus is a communication bus used to communicate between the BECM and the hybrid battery interface control modules inside the hybrid battery pack assembly. The BECM dedicated bus does not communicate with the scan tool.

This diagnostic is used to check the internal BECM dedicated bus off. If the transmit error counter of a bus is greater than 255 (carry condition in case of an 8-bit transmit error counter) then the supervisor requests the physical layer to set the bus into the "Bus Off" state. At this time, the battery energy control module increases its fail count. When this fail count is over X Cal Value (7) during Y Cal Value(10 samples), this fail code will happen.

Conditions for Running the DTC

* Vehicle ON.
* The system voltage is at least 9 V.

Conditions for Setting the DTC

The control module setting the DTC has received too many transmit errors on the BECM dedicated serial data circuits.

Action Taken When the DTC Sets

* DTC U2401 is a type A DTC.
* The battery energy control module sends all hybrid battery interface control module data to Invalid, 0 V, and -40°C (-40°F) on high voltage energy high speed GMLAN bus.

Conditions for Clearing the DTC

* DTC U2401 is a type A DTC.
* No bus off status.

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 control module on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more control modules was interrupted during diagnosis.
- A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and control modules 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 control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
* The engine will not start when there is a total malfunction of the BECM dedicated bus.
* Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) with the scan tool.

Reference Information
Schematic Reference

* Data Communication Schematics [1][2]Electrical Diagrams
* Hybrid Energy Storage Schematics Hybrid Energy Storage Schematics
* Control Module References Programming and Relearning

Connector End View Reference

Component Connector End Views Hybrid/EV Battery Pack X1
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

Scan Tool Reference

Control Module References Programming and Relearning for scan tool information
Special Tools

EL-48900 - HEV Safety Kit

For equivalent regional tools, refer to Special Tools Tools and Equipment.

Circuit/System Testing

Danger: Always perform the High Voltage Disabling procedure prior to servicing any High Voltage component or connection. Personal Protection Equipment (PPE) and proper procedures must be followed.


The High Voltage Disabling procedure will perform the following tasks:


* Identify how to disable high voltage.

* Identify how to test for the presence of high voltage.

* Identify condition under which high voltage is always present and personal protection equipment (PPE) and proper procedures must be followed.


Before working on any high voltage system, be sure to wear the following Personal Protection Equipment:


* Safety glasses with appropriate side shields when within 15 meters (50 feet) of the vehicle, either indoors or outdoors.

* Certified and up-to-date Class "0" Insulation gloves rated at 1000V with leather protectors.


- Visually and functionally inspect the gloves before use.

- Wear the Insulation gloves with leather protectors at all times when working with the high voltage battery assembly, whether the system is energized or not.


Failure to follow the procedures exactly as written may result in serious injury or death.


Danger: The Volt Battery Pack will utilize an exchange program. Please consult the most recent revision of bulletin/PI #PIP4841, available in Service Information (SI), for a list of approved Volt Battery Pack service procedures. Components that may be removed and serviced without exchanging the complete battery pack are identified in the bulletin/PI. Please contact the GM Technical Assistance Center (1-877-446-8227) if you have any questions.


Note: Each control module may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following:

* Control modules connected to the BECM dedicated serial data circuits
* BECM dedicated serial data circuit terminating resistors
* Control module locations on the BECM dedicated serial data circuits
* Each control module 5V reference, low reference, and BECM dedicated serial data circuit terminals

1. Perform the High Voltage Disabling procedure before proceeding with this diagnostic. Refer to High Voltage Disabling High Voltage Disabling.
2. Reconnect the 12 V battery.
3. Vehicle OFF, disconnect the harness connectors with the BECM dedicated serial data circuits at an easily accessible control module that is not communicating.
4. Vehicle in Service Mode, test for less than 4.5 V between each BECM dedicated serial data circuit at the control module that was just disconnected and ground.

• If one or more BECM dedicated serial data circuits are greater than the specified range, test the BECM dedicated serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuits for a Short to Voltage.

5. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 100 ohm between each BECM dedicated serial data circuit at the control module connector that was just disconnected and ground.

• If one or more BECM dedicated serial data circuits are less than the specified range, test the BECM dedicated serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to Ground.

6. Test for 110-130 ohm between the BECM dedicated serial data circuits at the control module that was just disconnected.

• If less than 110 ohm, test for a short between the BECM dedicated serial data circuits. Refer to Testing the Serial Data Circuits for a Short Between The Circuits.
• If greater than 130 ohm, test the BECM dedicated serial data circuit for an open/high resistance. Refer to Testing the Serial Data Circuits for an Open/High Resistance.

7. If all serial data circuits test normal, refer to Testing the Control Module Circuits.
8. If all circuits test normal, replace the control module that was just disconnected.

Note: Some hybrid/EV battery interface control modules have a loop in the harness that connects its 5 V reference circuit back to another control circuit of the same control module. 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 control module for each of the following tests.

Testing the Serial Data Circuits for a Short to Voltage
1. Vehicle OFF, disconnect the harness connectors with the BECM dedicated serial data circuits at another control module that is not communicating, in the direction of the circuit shorted to voltage.
2. Vehicle in Service Mode, test for greater than 4.5 V between each BECM dedicated serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more BECM dedicated serial data circuits are greater than 4.5 V.

• If all BECM dedicated serial data circuits are less than the specified range, replace the control module that was just disconnected.

3. Repeat step 1 until one of the follow conditions are isolated:

* A short to voltage on the BECM dedicated serial data circuit between 2 control modules or splice packs, if equipped.
* A short to voltage on the BECM dedicated serial data circuit between a control module and a terminating resistor.

Testing the Serial Data Circuits for a Short to Ground
1. Vehicle OFF, disconnect the harness connectors with the BECM dedicated serial data circuits at another control module that is not communicating, in the direction of the circuit shorted to ground.
2. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k ohm between each BECM dedicated serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more BECM dedicated serial data circuits are less than 1k ohm.

• If all BECM dedicated serial data circuits are greater than the specified range, replace the control module that was just disconnected.

3. Repeat step 1 until one of the follow conditions are isolated:

* A short to voltage on the BECM dedicated serial data circuit between 2 control modules or splice packs, if equipped.
* A short to voltage on the BECM dedicated serial data circuit between a control module and a terminating resistor.

Testing the Serial Data Circuits for a Short Between The Circuits
1. Connect the harness connectors at the control module that was previously disconnected.
2. Vehicle OFF, disconnect the harness connectors with the BECM dedicated serial data circuits at another control module that is not communicating, in the direction of the circuits shorted together.
3. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 ohm between each pair of BECM dedicated serial data circuits at the control module connector that was just disconnected. Verify that one pair of BECM dedicated serial data circuits are less than 110 ohm.

• If each pair of BECM dedicated serial data circuits is greater than the specified range, replace the control module that was just disconnected.

4. Repeat step 1 until one of the following conditions are isolated:

* BECM dedicated serial data circuits shorted together between 2 control modules or splice packs, if equipped
* BECM dedicated serial data circuits shorted together between a control module and a terminating resistor

Testing the Serial Data Circuits for an Open/High Resistance
1. Connect the harness connectors at the control module that was previously disconnected.
2. Vehicle OFF, disconnect the harness connectors with the BECM dedicated serial data circuits at another control module that is not communicating, in the direction of the open circuit.
3. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 ohm between each pair of BECM dedicated serial data circuits at the control module connector that was just disconnected. Verify that one pair of BECM dedicated serial data circuits are greater than 130 ohm.

• If each pair of BECM dedicated serial data circuits is less than the specified range, replace the control module that was just disconnected.

4. Repeat step 1 until one of the following conditions are isolated:

* An open/high resistance on the BECM dedicated serial data circuit between 2 control modules or splice packs, if equipped
* An open/high resistance on the BECM dedicated serial data circuit between a control module and a terminating resistor
* An open/high resistance terminating resistor

Testing the Control Module Circuits
1. Vehicle OFF, disconnect the harness connectors at an easily accessible control module that is not communicating.
2. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 10 ohm between the low reference circuit terminal at the control module that was just disconnected and ground.

• If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuits test normal, replace the appropriate control module.

3. Vehicle in Service Mode, test for 4.8-5.2 V between each 5 V reference circuit terminal and ground, at the control module connector that was just disconnected.

• If less than the specified range, test the 5 V reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the appropriate control module.
• If greater than the specified range, test the 5 V reference circuit for a short to voltage. If the circuit tests normal, replace the appropriate control module.

4. Connect the harness connectors at the control module that was just disconnected.
5. Repeat step 1 at another control module until a short to voltage, a short to ground, or an open/high resistance is found on the control module circuits.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.

* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Programming and Relearning for control module replacement, programming and setup

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