U0101: Lost communication with transmission control module (TCM)

What U0101 means

The transmission control module (TCM) manages gear changes on an automatic or automated manual gearbox and shares data with the engine ECU over the network. Another unit has stopped receiving its messages. Without that information, the gearbox usually locks into one gear to protect itself.

Most common causes

• Weak battery or loose terminals
• Gearbox ECU supply fuse or relay
• Gearbox ECU connector loose, corroded or contaminated with oil or water
• Damaged CAN wiring to the gearbox
• Failed gearbox ECU, sometimes from heat (often mounted on the gearbox)

Symptoms

• Gearbox stuck in one gear or limp mode
• Engine or gearbox warning light on
• Gear indicator missing from the dashboard
• Harsh shifts or no reverse

Can you drive with U0101?

With care You can usually drive gently to a garage, even if the gearbox holds a single gear. Avoid motorways and steep hills if you only have one gear.

How to check it

See whether the gearbox ECU shows up when you scan: if it does not respond at all, suspect its supply, earth or connector. Check the battery and gearbox fuses from the handbook first.

How to read and clear U0101 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see U0101 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

• U0100 Lost communication with engine control module (ECM/PCM A)
• U0102 Lost communication with transfer case control module
• U0103 Lost communication with the gear shift module
• U0104 Lost communication with the cruise control module
• U0107 Lost communication with the throttle actuator control module
• U0121 Lost communication with the ABS control module
• U0126 Lost communication with the steering angle sensor module
• U0128 Lost communication with the park brake control module
• U0131 Lost communication with the power steering control module

U0101 is a generic code, so it means the same on any make, for example on Abarth, Cupra, Hyundai, Jeep, Mini, Seat. See what Final OBD does on your car in makes and models.