U0101 – Data bus: transmission control module (TCM) – no communication

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By Jason (Contact Me)
Last Updated 2016-08-29
The Automotive Copywriter
CodeFault LocationProbable Cause
U0101 Data bus: transmission control module (TCM) - no communication
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Table of Contents

  1. What Does Code U0101 Mean?
  2. What are the common causes of code U0101?
  3. What are the symptoms of code U0101?
  4. How do you troubleshoot code U0101?
  5. Codes Related to U0101
  6. Get Help with U0101

What Does Code U0101 Mean?

In vehicles with electronically-controlled transmission shifting, a module is used to control when the transmission shifts and which gear should be engaged. The module modifies voltage to individual shift control solenoids which in turn direct fluid pressure to the proper channels according to the shift lever position and the speed. If DTC U0101 is present, there has been a loss of communication with the Transmission Control Module (TCM), either current or history. It can be a fault between two modules or a communication failure on the CANbus line. Because the CANbus line is used for two-way communication, a disruption can mean that solenoid positions are not known and the transmission will resort to default positions. Often, when a fault occurs that sets DTC U0101 and subsequently clears up, cycling the ignition may restore the vehicle's normal function.  

What are the common causes of code U0101?

  • Broken or damaged wiring (sometimes rodent-related)
  • IOD fuse poor connection
  • Low battery voltage
  • Transmission control module internal short
  • Bent connector pins
  • Corrosion on wiring or connectors

What are the symptoms of code U0101?

  • Check Engine light illuminated
  • Transmission shifts erratically
  • Transmission stuck in 2nd gear or limp mode
  • PRNDL display inaccurate or not working
  • Diagnostic scan tool won't power up

How do you troubleshoot code U0101?

Before attempting to determine the fault causing DTC U0101, determine if there is a service bulletin from your vehicle manufacturer. Check with your dealership or parts retailers for information pertaining to your specific vehicle. You can save hours of trying to diagnose the problem. If the PCM has been replaced, verify that the VIN is correct in the module. Also, low battery voltage can set an erroneous DTC U0101. Another common concern is a poor connection at the Ignition Off-Draw (IOD) fuse. Make sure the IOD fuse is not burnt and is making a good connection. A quality scan tool and a digital volt-ohmeter (DVOM) are necessary to trace U0101 issues. Because DTC U0101 is refers to loss of communication with the TCM, determine if your scan tool can communicate with the TCM. If the scan tool won't power up, check if your data link connector (DLC) fuse is burnt. With your scan tool, check if the TCM is active on the network. If the TCM is active, your condition is most likely intermittent, which can be hard to trace. For stored codes in this situation, clear the codes and road test, then recheck if the codes return. If not, the fault may not require repair at this time. Most commonly, chafed, broken, or corroded wires are the cause of DTC U0101. 300 tcm For active faults (inactive TCM on network), begin by checking the main connector at the TCM. Some vehicles have a standalone TCM located in the engine bay, and others are integral on the transmission itself. Probe the connector for 12V power at the location indicated on your wiring diagram, and verify the ground is proper as well. If 12V power is not present, check any related fuses, then repair the circuit as required. Remove the connector and inspect for corrosion, moisture, and bent pins. Repair as necessary, then reconnect and recheck for codes. If your vehicle has a standalone TCM, check the connector on the transmission in the same way. When you reattach the connector, make sure to install it squarely to prevent pin damage and secure it well using the factory method. Check the CANbus circuit for resistance if the code is still present. Using your DVOM, probe the CANbus circuit on the data link connector under your dash. Typically, pins 6 and 14 are correct. You should read steady resistance of 60 ohms +/-1 ohm. Wiggle along the CANbus circuit from the DLC to the transmission. Note any variations in the resistance as you wiggle the wires, indicating the location of your problem. If you find cracked wiring insulation, a broken wire, or chafing insulation, repair it with a soldered connection protected by weather-tight heat shrink. If the resistance is 120 ohms, you have an open circuit on your bus line that needs to be repaired. Check all wiring connectors for bent pins, corrosion and moisture along the circuit and repair as necessary. Protect all connections with dielectric grease and ensure the connectors lock together well. If you don't find a problem with the wiring, there may be an internal failure with the TCM. It's best to have the dealership or qualified mechanic diagnose and replace the TCM as a replacement module requires programming in most cases before it will operate.  
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