U0121
U0121-00-LOST COMMUNICATION WITH ANTI-LOCK BRAKE SYSTEM (ABS) CONTROL MODULE
For a complete wiring diagram, refer to the Wiring Information.
Theory of Operation
The primary communication network between electronic control modules is the Controller Area Network (CAN) data bus system. The Controller Area Network (CAN) data bus allows all electronic modules connected to the bus to share information with each other. Regardless of whether a message originates from a module on the higher speed CAN C (500K) Bus or on the lower speed bus, CAN B (50K) Bus, the message structure and layout is similar, which allows the Body Control Module/Central GateWay (BCM or BCMCGW) to process and transfer messages between the CAN buses. The BCM stores Diagnostic Trouble Codes (DTCs) for certain bus network faults on the CAN C (500K) Bus and CAN B (50K) Bus.
All modules transmit and receive messages over one of these buses. Data exchange between the modules is achieved by serial transmission of encoded data messages (a form of transmission in which data bits are sent sequentially, one at a time, over a single line). Each module can both send and receive serial data simultaneously. Each data bit of a CAN Bus message is carried over the bus as a voltage differential between the two bus circuits which, when strung together, form a message. Each module uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the same time. Corruption of a single bit within a message will corrupt the entire message. Each message contains a Cyclic Redundancy Check (CRC) which specifies the message size exactly. If the message detected conflicts with the CRC the ECU receiving it will determine the message to be an error and consider that communication has not been possible. Diagnosis of this condition using a lab scope may reveal activity that appears to be Bus data messages even if no actual communication is possible. Communication problems that affect the whole bus, as a result of opens and terminal push outs are more likely to occur on data busses that operate at a high speed than a data bus that operates at a lower speed.
When an open circuit or terminal push out occurs one or more modules can become isolated from the remainder of the bus. The isolated module will attempt to communicate, but will not be able to receive messages or determine arbitration from other modules. Each time the isolated module attempts to communicate it alters the bus voltage on the intact bus circuit. Without functioning arbitration the isolated module alters the bus voltage while other bus messages are being sent thereby corrupting the messages on the remainder of the bus.
The CAN C bus modules are connected in series, and the CAN B bus modules are connected in parallel. Both use a two-wire bus using a twisted pair, where the wires are wrapped around each other to provide shielding from unwanted electromagnetic induction, thus preventing interference with the relatively low voltage signals being carried through them. While the CAN bus is operating (active), one of the bus wires will carry a higher voltage and is referred to as the CAN High or CAN bus (+) wire, while the other bus wire will carry a lower voltage and is referred to as the CAN Low or CAN bus (-) wire.
- When Monitored:
With the ignition on for at least five seconds.
Battery voltage between 10 and 16 volts.
Body Control Module (BCM) is configured correctly.
- Set Condition:
Bus messages not received from the Anti-Lock Brake Module (ABS) for approximately two to five seconds.
1. VERIFY DTC IS ACTIVE
NOTE: Make sure the battery voltage is between 10 and 16 volts before proceeding.
1. With the scan tool, read active DTCs.
Is this DTC active?
Yes
- Go To 2
No
- Perform the STORED LOST COMMUNICATION DTCS diagnostic procedure Stored Lost Communication DTCs.
2. CHECK FOR ANY OF THE FOLLOWING ACTIVE DTCS
1. With the scan tool, read all active DTCs from all modules.
NOTE: Check for BCM configuration, CAN C hardware electrical, VIN Missing/Mismatch, battery or ignition related DTCs.
Does the scan tool display any active DTCs to the conditions listed above?
Yes
- Go to and perform the appropriate diagnostic procedure.
No
- Go To 3
3. VERIFY THE ABS IS ACTIVE ON THE BUS
1. Turn the ignition on.
2. With the scan tool, verify the ABS is active on the bus.
Is the ABS active on the bus?
Yes
- Go To 4
No
- Perform the NO RESPONSE FROM ABS diagnostic procedure No Response From ABS (Antilock Brake Module).
4. CHECK FOR ADDITIONAL COMMUNICATION RELATED DTCS
1. With the scan tool, read all the DTCs.
Is there more than one module with active DTCs "Logged Against" the ABS?
Yes
- Replace/update the ABS module. Anti-Lock Brake System Module - Removal.
- Perform the ABS VERIFICATION TEST. ABS Verification Test.
No
- Go To 5
5. CLEAR DTC IN MODULE SETTING FAULT
1. With the scan tool, select the module setting the DTC against the ABS.
2. Clear active DTCs.
Is this DTC still active?
Yes
- Replace/update the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Go To 6
6. CYCLE IGNITION
1. Cycle the ignition from RUN to OFF three times ending with the key in the RUN position.
Does this DTC become active?
Yes
- Go To 7
No
- The condition is not present at this time. Using the wiring diagram/schematic as a guide, inspect the wiring for chafed, pierced, pinched, and partially broken wires and the wiring harness connectors for broken, bent, pushed out, and corroded terminals.
- Perform the appropriate verification test for the module. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.
7. CHECK FOR ADDITIONAL LOST COMMUNICATION FAULTS
1. With the scan tool, view the modules.
Does the BCM and other CAN C BUS modules show Lost Communication with the remaining CAN C BUS modules?
Yes
- Go To 8
No
- Replace/update the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.
8. CHECK THE CAN C BUS (+) CIRCUIT FOR A SHORT TO VOLTAGE
1. Measure the voltage between the CAN C Bus (+) circuit and ground.
Is the voltage above 5.0 volts?
Yes
- Repair the short to voltage in the CAN C Bus (+) circuit. The short to voltage is present in one of the CAN C Bus circuits. Use the Module Connectors to isolate portions of the CAN C Bus in order to locate the concern.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Go To 9
9. CHECK THE CAN C BUS (-) CIRCUIT FOR A SHORT TO GROUND
1. Turn the ignition off.
2. Measure the resistance between ground and the CAN C Bus (-) circuit.
Is the resistance below 10k Ohms?
Yes
- Repair the short to ground in the CAN C Bus (-) circuit. The short to ground is present in one of the CAN C Bus circuits. Use the Module Connectors to isolate portions of the CAN C Bus in order to locate the concern.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Replace/update the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.