U0101🔧 Vehicle network (CAN bus) and transmission computerDrive with caution, fix soon

U0101: Lost Communication With TCM (Meaning, Causes, Fixes)

Reviewed 2026-10-22 · Based on 2 cited source(s)

What U0101 means

The transmission control module (TCM) is the computer that runs the automatic transmission, using sensor inputs to control parts like the shift solenoids and torque converter clutch (RepairPal). It shares information with the car's other computers over a two-wire network called the controller area network (CAN bus). U0101 sets when the TCM is not receiving or sending messages on that network. OBD-Codes.com calls it a generic communication code that applies to most makes, including Chevrolet, Cadillac, Ford, GMC, Mazda and Nissan, with troubleshooting that varies by manufacturer and wiring. RepairPal notes that on many vehicles battery voltage must be between 9 and 16 volts for the code to set, and that a technician should read about 60 ohms between pins 6 and 14 of the diagnostic port when the network's two terminating resistors are good. If several modules are reporting lost communication, RepairPal says the network is the likely problem rather than the TCM. The sources differ a little on causes: OBD-Codes.com says the most common failure is loss of power or ground and a faulty module is rare, while RepairPal lists a dead battery, a faulty TCM, the TCM circuit and the CAN bus without ranking them, and adds that reprogramming the TCM often fixes it. Related codes include U0100 (lost communication with the engine computer).

Severity: can I keep driving?

Drive with caution, fix soon

Get it diagnosed soon. RepairPal rates U0101 Moderate, meaning it may not threaten safety right away. OBD-Codes.com gives no rating but lists symptoms like the transmission not shifting or staying in one gear (usually 2nd or 3rd). If the car is stuck in one gear or shifting badly, drive gently and only as far as needed, since you may not have normal acceleration when merging or passing.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Loss of power or ground to the TCM, such as a blown fuse or dirty, loose ground; OBD-Codes.com says this is the most common failure
  2. An open in the CAN bus high (+) or low (-) wire (OBD-Codes.com, RepairPal)
  3. A short to power or to ground in either CAN bus wire (OBD-Codes.com)
  4. A weak or dead battery (RepairPal)
  5. A problem with the TCM circuit, such as damaged or corroded wiring and connectors (OBD-Codes.com, RepairPal)
  6. A faulty TCM or TCM software; OBD-Codes.com says a bad module is rare, while RepairPal lists a faulty TCM as a common cause and says reprogramming often solves it

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Check for other codes first. OBD-Codes.com says to diagnose any other network, battery or ignition codes before U0101 to avoid misdiagnosis.
  2. Make sure the battery is fully charged and its connections are clean and tight, then clear the code and see if it returns (RepairPal).
  3. Check the fuses that power the TCM (OBD-Codes.com).
  4. Find the TCM ground points and make sure they are clean and tight (OBD-Codes.com).
  5. Disconnect the negative battery cable, then inspect the TCM connector and wiring for rubbing, bare wires, burn marks, melted plastic or green corrosion on the terminals (OBD-Codes.com).
  6. Check for technical service bulletins (TSBs) for your vehicle that mention U0101 (OBD-Codes.com, RepairPal).

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The transmission won't shift or is stuck in one gear
  • A scan tool can't communicate with the TCM at all
  • Several modules are reporting lost communication, which calls for network testing with a multimeter or breakout box (RepairPal)
  • The TCM needs reprogramming or replacement, which needs dealer-level or professional scan tools (OBD-Codes.com, RepairPal)

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is U0101 serious?

RepairPal rates it Moderate. It can leave the transmission stuck in one gear or not shifting (OBD-Codes.com), so get it diagnosed soon and drive gently until then.

Can a bad battery cause U0101?

Yes. RepairPal says U0101 can result from a dead battery and that battery condition should be checked before other testing. On many vehicles the code only sets when battery voltage is between 9 and 16 volts.

Do I need a new transmission computer for U0101?

Not necessarily. OBD-Codes.com says loss of power or ground is the most common cause and a faulty module is rare. RepairPal says reprogramming the TCM often fixes it, and a replacement usually needs programming.

How much does it cost to fix U0101?

Neither source gives a repair cost. It can range from replacing a fuse or cleaning a ground to reprogramming or replacing the TCM, so get a diagnosis or a written estimate first.

Sources

  1. OBD-Codes.com: U0101 Lost Communication with Transmission Control Module (TCM) (Randy Worner, ASE Certified Master Technician)
  2. RepairPal: U0101 Lost Communication with TCM (rated Moderate)

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.