How to Fix U0101 Code: A Guide to Resolving the Transmission Control Module

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a new and old TCM image

Car issues are unavoidable most of the time. If you're a car owner, there's a big chance that you'll experience problems with your vehicle at some point or another. It could be something as simple as a flat tire or something more complex like transmission trouble.

If your Check Engine Light is illuminated, it could indicate something wrong with your vehicle. Code U0101 indicates a lost communication with the Transmission Control Module (TCM). If you see this code, don't panic, as it can be fixed!

What is Code U0101?

Code U0101 is an OBD-II trouble code for Lost Communication with Transmission Control Module (TCM). This error code means that the TCM and other control modules are not communicating.

The Controller Area Network (CAN) bus is the circuit used to communicate with the TCM. It is responsible for sending information between different modules in the vehicle. So, when it stops communicating with other control modules, your check engine light will come on and triggers code U0101.

Symptoms of Code U0101

As we all know, the control modules are very important in a vehicle. So, when the TCM is not able to communicate with other modules, you may experience these symptoms:

  • The Check Engine Light will come on.
  • Transmission shifting erratically
  • Delayed gear shifting
  • Engine stalling
  • Vehicle not starting
  • Stays in one gear

Causes of Code U0101

And, of course, when there are symptoms, there are causes. As mentioned above, transmission is one of the most important modules in a car. So, when there is an issue with communication between TCM and other modules, it could be due to:

  • Faulty wiring
  • CAN bus power short circuit
  • Poor battery voltage
  • Damaged connectors pins
  • Defective ECU
  • Faulty PCM (uncommon)

How to Diagnose Error Code U0101?

Now that you already know all the causes of this trouble code, you may proceed to diagnose. It is pretty straightforward, but you must have it done by professionals if you are not mechanically inclined. However, if you are comfortable fixing it yourself, let's get started!

Check the Technical Service Bulletin

One tip that could make your diagnosing easier is to check the (TSB) Technical Service Bulletins. It contains information on common problems and solutions that the company has already determined. You may see your problem and solution already listed there.

Scan for Other Trouble Codes

Get your diagnostic scan tool and scan for other codes. This will give you a better understanding of the problem. If any other codes are retrieved, address them first, as they may be the root cause of code U0101.

Perform a Visual Inspection

If you can't find any other codes, the next step is to perform a visual inspection. This means looking for damage or loose/harness wires. Visually inspect if they look burned or have a green tint indicating corrosion. If you find anything, fix it if possible or replace it if necessary.

Check all Fuses and Relays

Next, check all fuses and relays related to the TCM. These are usually located in the hood's Power Distribution Center (PDC). Look for burned-out or damaged items and replace them as needed.

Clear the Codes

Clear all the error codes from the modules saved from the computer's memory and see if the U0101 returns. You might need to do some advanced diagnosis and repairs if it does.

How to Fix Code U0101?

After all the diagnoses mentioned above, don't lose hope because you can still find other fixes. In most cases, this should take care of the problem. Code U0101 is usually caused by a dirty CAN bus. By taking the time to clean it, you can save money and any hassle in the long run.

Step-by-Step Guide to Cleaning the CAN bus

A dirty or faulty CAN bus is one of the most common causes of the U0101 code. So, if you have already checked all the connections and found no problems, then this is your next step.

  1. The first step is to locate the CAN bus. To do this, you will need to consult your vehicle's wiring diagram.
  2. Before starting, make sure to disconnect the negative battery terminal.
  3. Start by spraying the compressed air into all of the connectors on the CAN bus. Be sure to get in between all of the pins.
  4. Once you have finished with the compressed air, it is time to clean it with a cloth. Wipe down each connector until there is no residue left.

Step-by-Step to Testing the TCM Voltage

Before reconnecting the battery, take this chance to test the TCM voltage. You need to have a digital volt-ohmmeter (DVOM) in testing the voltage. To test:

  1. Attach the black lead of the DVOM to a good ground point on the car
  2. Next, connect the red lead of your voltmeter to battery positive (B+) and the black lead to each ground circuit.
  3. You should see the battery voltage at each ground circuit. If not, repair or replace the power or ground circuit.
  4. Find the CAN C+ (or HSCAN + circuit) and CAN C- (or HSCAN – circuit).
  5. Finally, Connect the red lead to CAN C+ with the black lead of your voltmeter connected to good ground. The standard volts should be 2.6 volts and fluctuate slightly with the key turned but with the engine off.

If you have finished all the diagnoses and repairs, but the code still returns, maybe it's time to replace your faulty TCM module. Replacing the TCM is not as straightforward as you think. So, if you feel you cant do it yourself, then it would be best to take it to a qualified mechanic.

But, if you want to try, there are some things that you should know before replacing the TCM:

  1. Before replacing the TCM module, ensure that you have the latest software for your vehicle.
  2. If you are unsure how to get the latest software for your car or truck, go to the nearest dealership, and they will help you out.
  3. Once you have downloaded the latest software, make a backup copy of it on a USB drive or external hard drive in case something goes wrong.

Now that you have everything ready, here are the steps to replacing the TCM:

  1. Disconnect the negative battery cable
  2. Remove the air cleaner assembly
  3. Disconnect the wiring harness from the TCM
  4. Remove two bolts that hold the TCM in place
  5. Replace with a new module and reattach the wiring harness and bolts
  6. Reconnect battery cable

Common Mistakes When Diagnosing the Code U0101

Making mistakes is a part of every diagnosing process. However, you should avoid things to save time and money. Here are some common mistakes people make when diagnosing code U0101 are:

  • Assuming that the problem is with something else besides the TCM, the code is specifically for "lost communication with TCM."
  • Not testing the power and ground circuits to the TCM
  • Not checking all fuses and relays that relate to the TCM
  • Replacing the PCM instead of the TCM (they are two different modules)

Code U0101 FAQs

1. Is the U0101 code serious?

Yes, this is one error code that needs to be addressed and diagnosed as soon as possible. This also requires automobile skills if you want to repair it yourself.

2. Can I drive with this code on my car?

If the code is U0101, then no, you should not drive. Many other codes will allow you to continue driving, but this one is not recommended because lost communication with TCM means that your car will not be able to shift gears properly.

3. What can I do to prevent code U0101 from coming back?

There is no clear answer to this question, but regularly inspecting your car could help. You can spot potential issues and have them fixed before they turn into something serious.

4. Should I fix this diagnostic trouble code immediately?

Yes, it is best to fix this issue as soon as possible to avoid any further damage to your car. Also, it will affect your emission test if you leave this issue unaddressed.

5. Do I need a special tool to diagnose and repair this error code?

Aside from the OBD scanner, you should also prepare basic automobile tools and a voltmeter.


Diagnosing and fixing the code U0101 can be a daunting task, but it is not impossible. By following our guide in this article, you will be able to get your car back on the road in no time. 

We hope this article has helped you understand what code u0101 means and how to fix it. 

If you are still having trouble fixing this code, feel free to reach out to us, and we would be more than happy to help!

Remember to regularly check your car for any potential issues to avoid any costly repairs in the future. Drive safe!