Fault codeU0101

MODERATE

Lost Communication With TCM

What this code means

The engine ECU is no longer talking to the transmission ECU. Often a wiring, connector or ECU problem.

U0101 is a generic code: it means the same thing on every make. U0101 belongs to family U01xx — Lost communication. Its page explains how to read a code and groups all its codes by part.

Communication between control units

A modern car has dozens of control units — engine, gearbox, ABS, airbags, dashboard — that talk over a network, mostly CAN. Each one checks that it regularly receives the messages of the others: a communication code means one unit has stopped hearing another, or receives messages it cannot trust.

What you notice

Several warnings at once, functions that stop working, sometimes a car that will not start; the fault may come and go.

How to check it

Start with the battery and its terminals: a voltage that drops too low during starting creates many of these codes at once. Then check the fuses and the connector of the unit that is no longer heard, and look for water or corrosion in the wiring. Codes recorded after a battery change or a jump start often clear by themselves.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Control module voltage

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

UNetwork
0standardised code
1Lost communication
01U0101