U0101 code: lost communication with the TCM

Code U0101 shows up when a computer in the car stops "hearing" the transmission module. Many people read it as "the TCM is fried", but carmakers say otherwise: the cause is usually the power feed, grounds, connectors, wiring or software. Here's what it means, which module sets it, what the car does, what to check first and when it really is the module.

What does code U0101 mean?

It's "Lost Communication With TCM": lost communication with the transmission control module. The car's computers send each other messages all the time over the CAN network; GM explains that each one sends a "state of health" message, and if a module loses power or ground, that message disappears. The computer left waiting stores U0101. GM is clear: the code doesn't mean the module that stored it has failed, and Nissan says that module is usually working fine.

Related communication codes

And the "-00" in U0101-00? It's the failure-type byte of the SAE J2012 standard: 00 means there's no further detail. Other numbers narrow down the diagnosis, like 87 (missing message) or 71 (invalid data); GM sends 00 to circuit testing and 71 and 72 to reprogramming.

Which computer sets U0101?

The one that stopped receiving the TCM, and it can be several: the engine computer (the most common; on PowerShift Fords the code is stored there), the instrument cluster (on the 2016 Civic CVT, Honda describes it as the cluster losing communication with the transmission), the ABS, the body module or the electronic shifter. That's why GM says to gather the codes from every module and note where each one is stored: if U0101 shows up in several computers at once along with other U codes, the likely cause is the network or the power feed, not one module.

U0101 symptoms

They depend on the car and on how often communication drops. These are cases carmakers document, in US bulletins unless noted:

• The transmission stays in one gear or won't shift: on the 6L80 Silverado and Sierra, GM documented it staying in 2nd; on the 2021 Charger, Challenger and 300 with the 3.6 engine, FCA documented it staying in 3rd.
• The car won't start or won't come out of Park: the 2016 and 2017 Civic CVT, the 2017 to 2020 F-150 and 2006 to 2008 Silverado and Sierra trucks with the Allison transmission.
• Loss of drive or power, sometimes a no-start, with the gear letter flashing on the cluster: the Focus and Fiesta PowerShift.
• Check engine light, service messages that come and go and erratic gauges: the 2015 to 2020 Trax, in a GM bulletin that includes Latin American countries.
• Transmission warnings: Chevrolet's Mexican manuals show "SERVICIO A LA TRANSMISIÓN" (service transmission), and on Volkswagens, if every lever position is highlighted on the cluster, the transmission is in its emergency program.

A transmission stuck in one gear doesn't prove it's U0101 or the module: that's what the computer does to protect it when it detects any fault. More in our guide to a transmission that won't shift.

What causes U0101?

• The battery and charging: Ford says to resolve any voltage problem before replacing the PowerShift TCM, and Nissan warns that low voltage can set CAN network codes.
• Fuses and the module's power feed: GM checks the underhood fuse block for damaged or loose terminals and mispositioned fuses.
• Grounds: GM says to check the TCM ground, and on the 2015 to 2020 Trax it found a poor ground, G103, on the left front frame rail in front of the battery.
• Connectors and terminals: on the 6T40, GM checks the transmission connector terminals; on the 6L80 it found backed-out terminals in the 16-way connector, a connector lock not fully seated and water; on the 2023 to 2025 Grand Cherokee, FCA says to look for corrosion on the valve body CAN terminals.
• The harness: GM looks for chafed harnesses, mainly on top of the transmission, and Ford found the transmission harness not properly secured on the 2017 to 2020 F-150.
• Software: FCA fixed U0101 on the 2021 Charger, Challenger and 300 with an engine computer update, and on the 2016 Grand Cherokee and Durango it updated the ABS, which set U0101 against the wrong module; GM reprogrammed the TCM on the 2023 Equinox and Silverado.
• A wrong or used module: GEARS magazine lists it among the causes, and GM warns that an incorrect module can add an extra resistor to the network.
• And last, the module itself: Ford replaced the PowerShift TCM after ruling out voltage, and Honda the 2016 and 2017 Civic CVT module for an internal fault.

U0101 by brand

What each carmaker documents for the most-searched cars in Mexico, in US bulletins unless noted:

How is U0101 diagnosed?

This is the order carmakers follow:

• Read the codes from every module, note where each one is stored and whether the scan tool can reach the transmission module. Nissan says to diagnose the communication codes first.
• Check whether the code is current or history only: GM says not to replace a module based only on a history U code, because a low battery or a module unplugged with the car on also leaves one.
• The battery and charging system.
• The module's fuses, power feeds and grounds, load-tested: GM wants less than 10 ohms on each ground.
• Connectors and terminals (loose, backed out, corroded or wet) and the harness routing.
• The CAN network, at the shop and with the car's wiring diagram: with the ignition off, about 60 ohms between terminals 6 and 14 of the diagnostic connector, from the two 120-ohm resistors at the ends of the network. GM accepts 50 to 70; 35 to 50 can mean an extra resistor, for example from a wrong module.
• Software updates for your VIN.
• Only then the module: GM says to replace the one that isn't communicating once every circuit tests good, and then it has to be programmed and the learn procedures run.

Be careful with the 60 ohms: it doesn't apply the same way on every car. Some have other networks on other terminals, resistors in the harness or a gateway module, and Nissan measures with the battery disconnected.

P0880 to P0884: the module's power feed

They aren't communication codes, but they often show up together: they're about the power reaching the transmission module or its solenoids, and they're checked the same way, starting with the battery, fuses and grounds:

• Hyundai and Kia: P0880 sets when solenoid supply voltage drops below 7 or rises above 22 volts; the transmission stays in 4th and bangs into D and R. Hyundai says to check the TCU1 and TCU2 fuses and the computer ground, and not to replace the transmission for this code (US bulletins).
• Mazda 3 with the 5-speed (FS5A-EL): P0882 is the module's back-up power supply below 2.5 volts, and Mazda starts with the 10 A ENG +B fuse, the battery terminal and the battery.
• Ford PowerShift: with P0882 (TCM power input low), Ford says to resolve the voltage before replacing the module.
• Chrysler, Dodge and Jeep: P0882 is the solenoid pack's power feed through the transmission relay, and P0884 a computer that restarted while moving; we cover them in our Chrysler code guide.

Can I drive with U0101?

Only as far as the shop, if the car allows it: with the transmission stuck in one gear you don't have every gear or full power, and since the fault is often intermittent, the car can lose drive or fail to restart, as Ford documented on the PowerShift.

How do you clear U0101?

With a scan tool, after the repair: if the fault is still there, the code returns. GM turns off the check engine light after 4 consecutive fault-free ignition cycles and erases the history after 50 cycles. Don't disconnect the battery to clear it: in a US bulletin, Volkswagen warns that the positive terminal can ground against the engine computer and damage it, and says to always disconnect the negative first.

Do I need to replace the transmission module?

Not right away. GM, FCA and Nissan solved many U0101 cases with wiring, grounds or reprogramming; Ford (PowerShift) and Honda (2016 and 2017 Civic) did replace the module. If it's replaced, it's programmed to the VIN and the learn procedures are run, and a used one only works if it's the same part and accepts the programming. We cover it in detail in our transmission control module guide.

TSS modules for the 6T40 and 6T45

TSS has rebuilt TEHCM modules for the 6T40 and 6T45 (Cruze, Sonic, Trax, Malibu, Equinox). They're programmed to the VIN, and it's worth replacing one only after ruling out wiring, fuses, grounds and software. On other cars, U0101 is almost always solved in the power feed, the network or the software, which aren't transmission parts:

See TSS modules

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