Pinpoint Test J: DTC U0121

Refer to MODULE COMMUNICATIONS NETWORK for schematic and connector information.

Normal Operation and Fault Conditions

The GSM and ABS module use information contained in messages on the HS-CAN2.

DTC Fault Trigger Conditions

DTC	Description	Fault Trigger Condition
GSM U0121:00	Lost Communication With Anti-Lock Brake System (ABS) Control Module 'A': No Sub Type Information	Sets in continuous memory in the GSM if data messages received from the ABS are missing for 5 seconds or longer.

Possible Sources

Communication concern
ABS
GSM

J1 VERIFY THE SCAN TOOL COMMUNICATES WITH THE ABS (ANTI-LOCK BRAKE SYSTEM) MODULE

Using a diagnostic scan tool, carry out the ABS module self-test.

Did the ABS module complete the self-test?

Yes

GO to J2

No	To diagnose an ABS module does not communicate with the scan tool concern, REFER to: Controller Area Network (CAN) Module Communications Network .

J2 RETRIEVE ALL ABS (ANTI-LOCK BRAKE SYSTEM) MODULE DIAGNOSTIC TROUBLE CODES (DTCS)

Using a diagnostic scan tool, retrieve all diagnostic trouble codes (DTCs) from the ABS self-test.

Are any non-network diagnostic trouble codes (DTCs) present?

Yes	DIAGNOSE all non-network diagnostic trouble codes (DTCs) first. REFER to: Anti-Lock Brake System (ABS) and Stability Control .

GO to J3

J3 CHECK THE GSM (GEAR SHIFT MODULE) FOR DIAGNOSTIC TROUBLE CODES (DTCS)

NOTE: If a GSM self-test is carried out, the driver door must be fully closed and locked. A false P166B DTC may set if the driver door is not fully closed and locked.
- Using a diagnostic scan tool, carry out the GSM self-test.
Are any non-network diagnostic trouble codes (DTCs) present?

Yes DIAGNOSE all non-network diagnostic trouble codes (DTCs) first. REFER to Diagnostic Trouble Code (DTC) Chart .

No GO to J4

J4 CHECK THE GWM (GATEWAY MODULE A) FOR DIAGNOSTIC TROUBLE CODES (DTCS)
- Using a diagnostic scan tool, check the GWM Continuous Memory Diagnostic Trouble Codes (CMDTCs).
Are any GWM diagnostic trouble codes (DTCs) present?

Yes REFER to: Controller Area Network (CAN) Module Communications Network .

No GO to J5

J5 RECHECK THE GSM (GEAR SHIFT MODULE) DIAGNOSTIC TROUBLE CODES (DTCS)

NOTE: If a GSM self-test is carried out, the driver door must be fully closed and locked. A false P166B DTC may set if the driver door is not fully closed and locked.

Using a diagnostic scan tool, clear the GSM diagnostic trouble codes (DTCs).

Road test the vehicle.

Using a diagnostic scan tool, carry out the GSM self-test.

Is DTC U0121 still present?

Yes

GO to J6

No	The system is operating correctly at this time. The DTC may have set due to high network traffic or an intermittent fault condition.

J6 CHECK FOR DTC (DIAGNOSTIC TROUBLE CODE) U0121 SET IN OTHER MODULES
- Using the diagnostic scan tool, perform the self-test for all modules.
- Retrieve the diagnostic trouble codes (DTCs) from all modules.
Is DTC U0121 present in other modules?

Yes GO to J7

No GO to J9

J7 CHECK FOR OTHER CAUSES OF NETWORK COMMUNICATION CONCERNS

NOTE: If new modules were installed prior to the DTC being set, the module configuration can be incorrectly set during the PMI or the PMI may not have been carried out.

CHECK the vehicle service history for recent service actions related to the ABS. If recent service history is found:
- verify correct replacement module was installed
  - HVBOM may be used to verify correct part application
- verify the configuration of replacement module was correct
  - re-configure module using as-built data if prior configuration is suspect
- verify the module was not obtained from a like vehicle and installed into customer vehicle
  - return the swapped module to source vehicle and obtain new replacement module

Operate the system and determine if the concern is still present.

Is the concern still present?

Yes

GO to J8

No	The system is operating correctly at this time. The concern may have been due to incorrect parts replacement procedures or incorrect module configuration.

J8 CHECK FOR CORRECT ABS (ANTI-LOCK BRAKE SYSTEM) MODULE OPERATION

Ignition OFF.

Disconnect and inspect the ABS module and related in-line connectors.

Repair:
- corrosion (install new connector or terminals - clean module pins)
- damaged or bent pins - install new terminals/pins
- pushed-out pins - install new pins as necessary

Reconnect the ABS module and all related in-line connectors. Make sure they seat and latch correctly.

Operate the system and determine if the concern is still present.

Is the concern still present?

Yes

CHECK OASIS for any applicable service articles: TSB, GSB (general service bulletin), SSM (special service message) or FSA (Field Service Action). If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions. If no service articles address this concern, INSTALL a new electric brake booster. REFER to: Electric Brake Booster (EBB) .

No	The system is operating correctly at this time. The concern may have been caused by module connections. ADDRESS the root cause of any connector or pin issues.

J9 CHECK FOR CORRECT GSM (GEAR SHIFT MODULE) OPERATION

Ignition OFF.

Disconnect and inspect GSM C2532 and all related in-line electrical connectors.

Using a good light source, inspect all disconnected electrical connectors for the following:
- corrosion - install new connector or terminal and clean the module pins
- damaged or bent pins - install new terminals or pins
- pushed-out pins - install new pins as necessary
- spread terminals - install new terminals as necessary

Reconnect GSM C2532 and all related in-line electrical connectors. Make sure they seat and latch correctly.

Using the diagnostic scan tool, clear all DTCs.

Carry out the GSM self-test.

Is DTC U0121 still present?

Yes

No	The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. ADDRESS the root cause of any connector or pin issues.