The ECU has stopped receiving messages from the transmission control module. The transmission falls into limp-home mode (typically locked in 3rd or 4th gear), the speedometer may stop working, and shift adaptation freezes.
Medium severity. Can I keep driving?Drive very moderately to a repair shop if the car is still moving, and avoid highways: it may stay in a fixed gear. If the transmission will not engage or it slips, do not continue and call a tow truck.
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Most likely causes
- TCM unpowered or grounded poorly
- CAN wiring to TCM broken
- Gateway module routing CAN messages incorrectly
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
What each character of U0101 means
- U: Communication network
- 0: Generic code: same on every make
- 1: Subsystem depends on the code range
- 01: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0101?
Drive very moderately to a repair shop if the car is still moving, and avoid highways: it may stay in a fixed gear. If the transmission will not engage or it slips, do not continue and call a tow truck.
What causes code U0101?
The most common causes are: tCM unpowered or grounded poorly; cAN wiring to TCM broken; gateway module routing CAN messages incorrectly.
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.