What does code U0101 mean?

Code U0101 stands for Lost Communication with TCM.

The transmission control module (TCM) is the computer that controls your vehicle’s transmission. Various sensors provide input to the TCM. It then uses this information to determine control of various outputs, such as the shift solenoids and torque converter clutch solenoid.

There are a number of other computers (referred to as modules) on board a vehicle. The TCM communicates with these modules over the controller area network (CAN) bus. CAN is a two-wire bus, consisting of the CAN High and CAN Low lines. There are two terminating resistors, one at each end of the CAN bus. These are needed to terminate communication signals which flow both ways.

Code U0101 indicates the TCM is not receiving or transmitting messages on the CAN bus.

U0101 symptoms

  • An illuminated check engine light
  • Transmission performance issues
  • Problems with the instrument cluster PRNDL display

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Common causes for U0101

Code U0101 is typically caused by one of the following:

  • A dead battery
  • A faulty TCM
  • A problem with the TCM circuit
  • A problem with the CAN bus

How to diagnose and repair U0101

Perform a preliminary inspection

Sometimes U0101 can pop up intermittently, or it can result from a dead battery. This is especially true if the code is a history code and not current. Clear the code and see if it returns. If it does, the next step is to perform a visual inspection. A trained eye can check for issues such as broken wires and loose connections. If a problem is found, the issue should be repaired and the code cleared. If nothing is discovered, check for technical service bulletins (TSBs). TSBs are recommended diagnostic and repair procedures put out by the vehicle manufacturer. Finding a related TSB can greatly reduce diagnostic time.

Check the battery

Proper battery voltage is crucial for control module operation. Before proceeding, battery condition should be checked. Recharge or replace the battery as needed and clear the codes.

Check for other DTCs

Diagnostic trouble codes (DTCs) set for multiple modules indicate there may be a problem with the CAN network. In this case, a professional will begin by checking the network instead of focusing on the TCM.

Like any other circuit, the CAN bus can be checked for shorts, high resistance and opens. This is typically done at the data link Connector with a digital multimeter (DMM). There are 16 pins on the data link connector, with pins 6 and 14 being CAN high and CAN low. The DMM is connected to one or both of these pins for testing.

The two CAN terminating resistors can also be checked with a DMM at the data link connector. A normal reading for the resistors is 60 ohms, with the meter connected between pins 6 and 14. If one of the resistors fails, the bus typically remains operational. However, if both fail, the bus shuts down.

A breakout box is an alternate tool that can be used for network testing. This tool connects directly to the data link connector and is to monitor network activity.

Check the TCM

If everything is good up to this point, it’s time to start checking the TCM. A technician will begin by attempting to communicate with the TCM using a diagnostic scan tool. The scan tool connects to the vehicle via the diagnostic port. Once it’s hooked up, it acts like just another module on the CAN bus.

If the TCM doesn’t respond to the scan tool, the next step is to figure out why. Like any other electrical device, the TCM circuit must have proper power and ground. TCM circuit integrity can be checked with a DMM. If a problem is found, the factory wiring diagram will need to be traced and the circuit repaired.

By now, if everything else checks out, it’s time to focus on the TCM. Before replacing the TCM, it’s software should be checked. Many times, simply reprogramming the TCM will solve the problem. If programming isn’t the issue, the TCM will need to be replaced. In most cases, the new TCM will need to be programmed after installation.

Other diagnostic codes related to U0101

All the ‘U’ codes are network communication codes. Codes U0100 to U0300 are lost communication with XX module codes.

Code U0101 technical details

On many vehicles, battery voltage must be between 9 - 16 volts for code U0101 to set.

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