What U0101 means
U means network (communication between modules). The 0 after it marks a generic code defined by SAE rather than by the manufacturer, and the U01xx block is reserved for lost-communication faults with specific modules.
Modern vehicles connect their control modules on a shared data network, most often CAN bus. Each module expects regular messages from the others. A "lost communication" code is set by a module that stopped receiving expected messages from the named module for longer than an allowed time.
The module named here is the transmission control module. What you notice depends on what that module does: shift problems or a transmission limp-home mode.
What “Lost Communication” means here
A module on the network expected periodic messages from the transmission control module and did not receive them for longer than allowed. The code is stored in the module that noticed the silence, not necessarily in the one that went quiet, and the named module may itself be unpowered, faulty, or cut off by wiring.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany U0101:
- Warning lights for the system the missing module controls
- Features of that system not working
- Several U-codes stored at once in different modules
Causes to consider
- Disconnected TCM plug
- Corroded TCM connector pins
- Blown TCM fuse
Where to start diagnosing U0101
- Check battery voltage and the charging system first; low voltage causes many communication faults.
- Check whether the named module is powered: its fuses, power feeds and grounds.
- Scan all modules: if one module is silent to every other module, focus on it; if many modules are missing, suspect the network wiring.
- Inspect network wiring and connectors near the named module; on CAN, the two termination resistors should measure about 60 Ω together across the bus pins with the battery disconnected.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to U0101. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Control module voltage (PID 0x42) | Low system voltage can cause communication faults. |
| Module list | A full network scan showing which modules respond. |
Related codes
Codes from the same vehicle network communication family. If several are stored together, the shared network wiring, power supply or ground is the place to start.
| Code | Definition |
|---|---|
| U0100 | Lost Communication With ECM/PCM "A" |
| U0102 | Lost Communication With Transfer Case Control Module |
| U0103 | Lost Communication With Gear Shift Control Module "A" |
| U0104 | Lost Communication With Cruise Control Module |
| U0105 | Lost Communication With Fuel Injector Control Module |
| U0106 | Lost Communication With Glow Plug Control Module |
| U0107 | Lost Communication With Throttle Actuator Control Module |
| U0108 | Lost Communication With Alternative Fuel Control Module |
Further reading
Symptoms that lead to U0101
If you have not scanned the car yet, or U0101 is one of several codes stored, these guides start from what the vehicle is actually doing.
Frequently Asked Questions
What does U0101 mean?
U0101 is the generic SAE J2012 code for "Lost Communication with Transmission Control Module (TCM)". In plain terms, it means another module on the vehicle network stopped hearing from the transmission control module.
Can I keep driving with U0101?
Depends entirely on the module that has gone silent. If it controls braking, stability or steering, do not drive until it is checked.
What should I check first for U0101?
Check battery voltage and the charging system first; low voltage causes many communication faults.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.