What does U0101 mean?

Other modules can no longer communicate with the transmission control module.

Why does the car stay in one gear with U0101?

Without the TCM the transmission falls back to a limp mode to protect itself.

Network communication

U0101 — Lost Communication With TCM

Another control module, usually the engine ECU, cannot communicate with the transmission control module (TCM). Gear changes may become harsh or the transmission may go into limp mode.

Severity: High

Common causes

CAN bus wiring fault between the modules
Faulty TCM or its power and ground supply
Blown TCM fuse or relay
Corroded connector or water damage
Low battery voltage

Symptoms

Transmission stuck in one gear (limp mode)
Harsh or no shifts
Gearbox warning light and check engine light
Other U codes

How to check

Check battery voltage and the TCM fuse, relay and ground
Check CAN wiring and connectors at the TCM
Scan the vehicle to see which modules can still communicate
Test the module power supply and replace or repair the TCM if the rest is good

Can you keep driving?

Not recommended for long: limp mode and harsh shifting can damage the transmission. Drive only to a workshop.

Frequently asked questions

What does U0101 mean?: Other modules can no longer communicate with the transmission control module.
Why does the car stay in one gear with U0101?: Without the TCM the transmission falls back to a limp mode to protect itself.

General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.