Theory of Operation
The Vehicle Speed Sensor (VSS) signal is transmitted to the Anti-Lock Brake System (ABS) Module via dedicated hard wire. The VSS signal is then transmitted over the Controller Area Network (CAN) bus to the Transmission Control Module (TCM).
The Output Speed Sensor signal is a direct input to the TCM.
The TCM monitors these signals to determine if the VSS signal CAN message is valid. If the VSS signal and the Output Speed Sensor signal do not match, this Diagnostic Trouble Code (DTC) is set.
When Monitored and Set Conditions
When Monitored:
This diagnostic runs continuously when the following conditions are met:
- Ignition on.
- The shifter is in a forward gear.
- No Antilock Brake System (ABS) or Output Speed Sensor Diagnostic Trouble Codes (DTCs) set.
Set Conditions:
- The Vehicle Speed Sensor (VSS) signal received via Controller Area Network (CAN) and the Output Speed Sensor signal values do not match.
Default Actions:
- Malfunction Indicator Lamp (MIL) is illuminated on the second consecutive trip that the diagnostic fails.
- The Transmission Control Module (TCM) will not request any positive torque interventions from the Powertrain Control Module (PCM) via CAN.
Possible Causes
CAN C BUS CIRCUIT OPEN OR SHORTED
ANTI LOCK BRAKE SYSTEM (ABS) MODULE
TRANSMISSION CONTROL MODULE (TCM)
Always perform the DIAGNOSTIC TROUBLESHOOTING PROCEDURE before proceeding. (Refer to DTC-Based Diagnostics/MODULE, Transmission Control (TCM)/Standard Procedure)(Refer To List 1).
Diagnostic Test
CHECK FOR AN ACTIVE DIAGNOSTIC TROUBLE CODE (DTC)
With the scan tool, read Transmission Control Module (TCM) DTCs and record on the repair order.
Record the Event Data and Environmental Data.
With the scan tool, erase DTCs.
Using the recorded Event and Environmental Data, along with the When Monitored and Set Conditions above, operate the vehicle in the conditions that set the DTC.
With the scan tool, read TCM DTCs.
Did the DTC return?
Yes
- Go To >>> (Lost communication check.)
No
- Go To >>> (Check wiring.)
Refer To List:
CHECK FOR ANTILOCK BRAKE SYSTEM (ABS) DTCS
With the scan tool, read ABS DTCs.
Is DTC U0121-Lost Communication With ABS Module set?
Yes
No
Refer To List:
CHECK WIRING AND CONNECTORS
The conditions necessary to set the DTC are not present at this time.
Using the schematics as a guide, inspect the wiring and connectors specific to this circuit.
Wiggle the wires while checking for shorted and open circuits.
Check for any Service Bulletins that may apply.
Were any problems found?
Yes
No
