Network communication (U-codes) · Network · generic (SAE-defined)
U0101: Lost Communication With TCM
U0101 means one or more modules stopped receiving network messages from the transmission control module (TCM).
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How U0101 sets
The engine control module and other modules expect regular data from the TCM over the controller area network (CAN). When those messages are missing for longer than allowed, a listening module stores U0101. Time limits and conditions are manufacturer-specific.
Common causes of U0101
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Low battery voltage
- Blown TCM fuse or failed power relay
- Corroded or loose TCM connector
- Damaged CAN wiring to the TCM
- Poor TCM ground
- Failed TCM, sometimes from heat or fluid intrusion
U0101 symptoms
- Check engine light on
- Transmission stuck in a default gear
- Harsh or no shifting
- Gear indicator blank or incorrect
- Reduced power in some vehicles
How U0101 is diagnosed
- Scan all modules and see if the TCM responds to the scan tool.
- Check battery and charging voltage.
- Check TCM fuses, power feed, and ground.
- Inspect the TCM connector and harness for corrosion, fluid, or damage.
- Test CAN wiring continuity between the TCM and the network.
- Check for technical service bulletins or software updates.
Typical fixes for U0101
- Repair battery or charging problems
- Replace blown fuses and repair the cause
- Repair TCM power, ground, or CAN wiring
- Clean or replace corroded connectors
- Replace and program the TCM if confirmed faulty
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
U0101 FAQ
What does code U0101 mean?
U0101 means one or more modules stopped receiving network messages from the transmission control module (TCM). The standard definition is "Lost Communication With TCM."
Can I drive with a U0101 code?
Limit driving until it is checked. The transmission may stay in a single default gear or shift harshly, which can make highway driving or merging unsafe.
Why is my transmission stuck in one gear with U0101?
When communication with the TCM is lost, the transmission often defaults to a fixed gear so the vehicle can still be moved to a repair shop.
Where is the TCM located?
It varies; some are mounted inside or on the transmission and others are in the cabin or engine bay, which affects how prone they are to heat or fluid damage.
Keep researching
More network communication (u-codes) codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.