U0101 means Lost Communication With TCM. A module on the vehicle network has stopped hearing from the transmission control module. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What U0101 means
Modern vehicles have many control modules that share information over a network, usually called the CAN bus. Codes starting with U mean a module has stopped hearing another one, and voltage and processor codes mean a module is not getting clean power or has an internal fault. Low battery voltage, poor grounds, blown fuses and damaged wiring cause many of these codes, so test the battery and charging system first.
U0101 symptoms
- Transmission stuck in one gear (limp mode)
- Harsh shifting
- Gear indicator blank or flashing
Common causes of U0101
- TCM without power or ground (fuse, relay or corroded ground)
- Damaged network wiring or connectors to the TCM
- Low battery voltage
- Failed TCM
How to diagnose U0101
- Check TCM fuses, power and grounds
- Inspect network wiring and connectors at the TCM
- Check whether a scan tool can communicate with the TCM
- Check battery and charging voltage
Can you drive with U0101?
Limited: the transmission is usually in limp mode. Drive only to a repair shop.
U0101 tips
A lost-communication TCM code often follows a jump-start, a low battery or a blown fuse. Check the TCM power and ground circuits and the network connector before assuming the module or the transmission has failed.
Will U0101 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing U0101 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| U0100 | Lost Communication With ECM/PCM A |
| U0073 | Control Module Communication Bus A Off |
| P0700 | Transmission Control System Malfunction |
Guides that help
- car battery life and voltage
- how to use an OBD2 scanner
- transmission won’t shift
- transmission control module
- limp mode
- transmission solenoid symptoms
When it is more than a sensor
Voltage and network codes come from wiring, the battery, the charging system or a module, not from engine or transmission wear. If a lost-communication code has left the transmission in limp mode, rule out the module and wiring before deciding the unit has failed, and see our transmissions only if testing confirms internal damage.
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