Modern vehicles have come a long way in terms of onboard diagnostic systems. Just about every module in your car is capable of running internal diagnostics and letting you know when something goes wrong. That being said, most check engine light codes people deal with are related to the engine. What many people don’t know is that your transmission can also report fault codes of its own.
Today we’ll take a look at one such code — the U0101 diagnostic trouble code. We’ll discuss what this code means, how you can pinpoint what caused it, and how to potentially fix it at home if possible.
What is a U0101 Fault Code?
The U0101 code means that the ECU has lost communication with TCM (Transmission Control Module). Modern vehicles that run automatic transmissions feature dedicated control modules whose job is to make sure that the transmission is working as intended. The TCM and the ECU are constantly communicating and exchanging information that is necessary to keep the vehicle running smoothly. Just like your brain needs to know what your arms are doing, the ECU needs to know what the TCM is doing, and in this case, vice versa.
When there’s an interruption in this line of communication, the vehicle usually slips into limp mode in order to protect itself from potential damage. The loss of communication with the TCM will trigger the U0101 code, thus letting you know that something is not right with communication between your transmission and the ECU.
Is U0101 a Standard Code?
The U1010 is a standard code since it begins with a 0. If you’ve been confused by the fact that this particular fault code doesn’t follow the usual P nomenclature, that’s because it’s not related to the powertrain in your car. Instead, the U designation means that the code is registering an issue in the communication between various modules in your car.
What Causes the U0101 Fault Code?
The silver lining with the U0101 code is that there are only so many things that can cause this error. Here’s what you’re most likely dealing with here:
- CAN bus ground issues
- CAN bus short
- Open CAN bus
- Failed TCM
More often than not, you’ll be dealing with the first three on the list. A bad TCM module is a rare occurrence, but it does happen. There’s also a chance that a larger CAN bus network problem is causing this error to pop up. We’ll discuss how to avoid chasing your tail diagnosing the U0101 error caused by a more severe CAN bus problem a bit later. For now, let’s talk about the symptoms you’ll notice when this DTC is triggered.
What Are the Symptoms of the U0101 DTC?
The list of actual symptoms caused by the loss of coms between the ECU and the TCM varies depending on your vehicle. In most cases, this error will push your car into limp mode..
Sometimes you might find that your car is refusing to shift gears altogether. It’s not uncommon for people to have their vehicles stuck in gear as long as U0101 is logged as a current error. Worst of all, you won’t always be stuck in a gear that’s practical, which often leads to you not being able to move the car.
How to Diagnose and Fix U0101 DTC?
Alright, so how do you go about diagnosing this error? This is where you have to put on your thinking hat and approach this problem in a forensic way. As it turns out, it’s often easy to go down the wrong rabbit hole and end up misdiagnosing the issue to a point where you risk losing time and money.
Run a Full Scan
First thing first, you’ll need to run a full scan and check for all DTCs. The reason why this step is so crucial is simple — a full diagnostic scan will help you exclude any other potential reasons why your ECU is unable to communicate with the TCM. That way you can focus on that part of your wiring harness and get to work.
Using an OBD II scanner will reveal another crucial piece of the puzzle. By attempting to connect to the TCM, you’ll either be able to establish communication, or you won’t. If your scanner can access TCM data, that means that you’re dealing with an intermittent issue. On the other hand, if you can’t talk to the TCM, your DTC is a current one. Knowing whether it’s a current or intermittent issue will come in handy later on.
Wiring Diagrams
Your car’s CAN bus network is massive. Wiring harnesses run through just about every nook and cranny of your car. Trying to isolate individual circuits is borderline impossible without a wiring diagram. If you’re looking to maximize your chances of fixing the issue behind this DTC, you’ll need to get your hands on a complete wiring diagram for your car.
This is especially true if you have to start probing the TCM connector. Knowing which wires are power wires is essential for the process.
Check The Grounds
With the electrical diagram in your hands, locate the TCM module and find all ground points. As it turns out, the most common cause of the U0101 error that’s CAN bus-related is a bad ground somewhere in the system. Locate your TCM ground points and inspected them thoroughly.
Check the Wiring Harness for Damage
Depending on the vehicle you’re working on, checking the wiring harness can be easy or extremely difficult. The idea is to look for obvious signs of damage caused by anything from rodents to exposure to water.
Most wiring harnesses use some kind of plastic shielding or wrapping as protection. Don’t go unwrapping these wiring bundles just yet. For now, visually inspect them for any obvious signs that there is a physical issue with the wires.
Inspect the Connector
Alright, the next step involves diagnosing the TCM connector. Disconnect the battery before you disconnect the connector. Then, with the TCM disconnected, start examining the connector for any obvious signs of damage. You’re looking for loose wiring, damaged wiring, corrosion, and similar issues.
Anything as subtle as a loose female connector pin is enough to cause an interruption in communication between the TCM and the ECU. You can check this by using the right gauge of wire. Simply find a wire that will fit inside the connector, and then test different pins. If all of them fit snug, but one feels loose, there’s a chance you’ve found your culprit.
If the visual inspection yields no results, it’s time to break out the big guns. You’ll need to have the connector powered for this part of the diagnostic process. You’ll also need some or all of the following tools
- An oscilloscope
- A multimeter
Using your wiring diagram, locate the TCM CAN bus wiring and place your oscilloscope probes on the corresponding wires. The oscilloscope should tell you if there’s an issue with any of these lines.
You can perform this test with a multimeter as well. Locate the battery power pins on the connector and check for battery power by placing the red probe on the pin and grounding the black probe of your multimeter. You should see the battery power on the display. Alright, now we know if the TCM is getting power, but what about the signal?
Locate the network wires (CAN + and CAN -) pins and test each by placing the red probe inside the pin and grounding the black probe. You should see values anywhere from 1.5V to 2.5V, usually closer to the latter.
One thing to remember, wiggle the connector whenever you’re probing it, especially if you’re dealing with an intermittent fault. There’s a chance that one of the wires isn’t seated properly, and you might catch it on your oscilloscope/multimeter when you wiggle the connector.
Check the TCM
If everything has checked out so far, you’re most likely looking at a faulty TCM. Unfortunately, there’s not much you can do about it aside from getting a new one. But wait, it gets better. You can’t just go buy a TCM and plug it into your car. No, you have to program it first. This is done either by a dealership or by 3rd party services.
We highly recommend that you leave buying a new TCM as the last resort solution. These things don’t really fail that often, and in most cases, the cause of all your troubles is bad wiring.
Can You Drive Your Car With U0101 Error?
You probably could, but it is definitely not recommended. Even though your transmission is most likely fine, it will most likely be in limp mode, or worse, stuck in gear. Driving around like that is only recommended if it means getting out of a bad situation. In all other scenarios, you shouldn’t be driving your car until you figure out what’s going on.
Get Quality Parts for Your Car
Speaking of transmissions, proper maintenance won’t solve your TCM issues, but it will prevent your transmission from developing mechanical issues. Here at eEuroparts.com, you’ll find the most extensive catalog of filters, transmission fluids, and other necessary items for a wide range of European makes and models.
To find what you’re looking for, simply select your vehicle using our vehicle selection tool, and make use of our search bar. You’ll be shown only products that are a guaranteed fit for your vehicle.
