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Generic SAE code Network communication

U0101: Lost Communication With TCM

Another module has stopped receiving messages from the transmission control module.

High risk

Have it checked soon. The gearbox may go into limp mode.

Short answer · Can I keep driving with U0101?

Usually possible for a short distance, but the gearbox may stay in one gear. Have it checked soon.

On this page

What does U0101 mean?

U0101 is stored when the engine control module or another module stops receiving messages from the transmission control module (TCM).

What the car does when the code is set

Other modules cannot receive data from the engine or transmission module. The engine may not start, the gearbox may go into limp mode and several warning lights can come on at once.

How the system works

The engine and transmission control modules share data with other modules over the network, such as engine speed, torque and gear. Each module expects regular messages; when they stop arriving for a set time, a lost communication code is stored.

UNetwork
0Generic code (SAE)
1Network communication
01Specific fault

Common symptoms

Check Engine LightAmber lightHigh riskTransmission Warning LightAmber lightHigh riskCar Won’t StartCheck soonNormal riskEngine Stalls While Driving Or IdlingCheck promptlyHigh riskCar In Limp ModeCheck promptlyHigh riskHarsh Or Delayed Gear ShiftsCheck soonNormal risk

Possible causes

Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.

  1. Power or ground fault at the moduleA blown fuse, failed relay or poor ground switches the module off the network.
  2. Wiring fault in the network linesDamaged or corroded CAN wires between modules.
  3. Low battery voltageVoltage drops during cranking can cause temporary lost communication.
  4. Failed control moduleLess common than wiring and power faults, but possible.
  5. Water damageModules and connectors exposed to water corrode.

How it is diagnosed

  1. Note how the car behavesIf the engine does not start or the gearbox is in limp mode, the module may really be offline. If everything works, the code may be an old, intermittent event.
  2. Check fuses, relays and groundsUse the fuse diagram for the engine or transmission control module.
  3. Check battery voltageCharge or test the battery before deeper diagnosis.
  4. Scan all modules and test the networkA workshop checks whether the module answers the scan tool and measures the CAN lines.

Questions about U0101

Why is the gearbox stuck in one gear?

Without the TCM, the gearbox may use a fixed limp-mode gear.

Does lost communication always mean a bad module?

No. Power, ground and wiring faults are more common causes than a failed module.

Terms on this page

ECU (Engine Control Unit)Transmission Control Module (TCM)OBD-IICar Battery