U0101: OBD-II code U0101 flags loss of communication with the transmission control module — the network can no longer see the TCM.

Symptoms

• Check engine light is on and the transmission enters limp mode, typically stuck in a single gear like 3rd or 4th.
• Harsh or delayed shifts, or the vehicle may refuse to shift out of park in some cases.
• The transmission gear indicator on the dashboard may go blank or show dashes because the cluster loses TCM data.

Common causes

• A failed TCM — the module itself is offline due to internal failure or water damage.
• A CAN bus fault (broken wire or short) between the TCM and the rest of the network.
• Loss of power or ground at the TCM — blown fuse or damaged wiring at the module connector.

What to do?

Confirm the TCM has power and ground at its connector. Check the TCM fuse. Measure CAN bus continuity to the TCM. If the TCM won't wake up despite good power and bus, replace and reprogram to the vehicle.