U0101: Lost Communication With TCM
Other modules have stopped receiving messages from the transmission control module over the vehicle network.
What U0101 means
U0101 means the engine computer, or another module on the network, has stopped hearing from the transmission control module. Without those messages, the engine computer cannot coordinate shifts, torque reduction or the gear display, and most transmissions fall back to a single default gear. KBB notes that driving is often possible but can lead to transmission and engine problems. The cause may be the TCM itself, but power supply problems are common: a weak battery, a blown fuse, a bad ground or corroded connector can all take the module off the network. On some vehicles the TCM is mounted inside the transmission or on top of it, where heat and fluid can damage it, and some makers have issued service bulletins on these units. Shops check the battery, fuses, power and ground at the TCM, then test the network wiring and try to communicate with the module directly. If it has power and good wiring but still will not talk, it is replaced and programmed.
Common symptoms
- Check engine light on
- Transmission stuck in one gear (limp mode)
- Harsh or no shifting
- Gear indicator blank or wrong
- Other warning lights
Most common causes
- Weak battery or low voltage
- Blown fuse or failed power feed to the TCM
- Bad ground at the TCM
- Damaged or corroded network wiring
- Failed transmission control module
- Water intrusion into the TCM or its connector
What to check yourself first
Have the battery tested and check the fuses that feed the transmission computer. Find out where your TCM is mounted; some sit in the engine bay or under a seat, while others are inside the transmission. Check visible connectors for corrosion or water.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace battery | $456-$493 | A weak battery or low voltage is a very common cause of communication codes. |
| Network (CAN bus) electrical diagnosis | $122-$179 | Network faults are usually found by checking power, ground and the two CAN bus wires at each module. |
| Replace transmission control module | $929-$1,006 | Only after power, ground and network wiring are confirmed good. |
| Transmission fluid change | $294-$356 | On TCMs mounted inside the transmission, overheated fluid can contribute to failures. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- U0100 Lost Communication With ECM/PCM "A"
- P0700 Transmission Control System (MIL Request)
- U0073 Control Module Communication Bus "A" Off
- U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module
- P0562 System Voltage Low