U0101 fault code: lost communication with the TCM
Plain-language definition
Lost Communication With TCM
What does it mean?
The transmission computer (TCM) controls an automatic or dual-clutch transmission and constantly exchanges data with the engine computer: engine speed, torque, accelerator pedal position and gear information are shared over the data network. U0101 means the module that stored the code (most often the engine computer) hasn't received the messages it expects from the transmission computer for a set time.
A one-time record can also appear when the battery is weak or the voltage drops. If the code keeps coming back, the problem is most often in the transmission computer's power supply, its connector or its data network connection. On some cars the transmission computer sits inside or on the transmission, so it can be affected by heat, oil and vibration.
You may notice the transmission switching to limp mode and staying in one gear, harsh gear shifts, the gear position not showing on the instrument cluster and several warning lights coming on. If it's a past record, there may be no symptoms.
How serious is it?
It's rated Warning, but when communication is lost, on many cars the transmission switches to limp mode and driving is noticeably affected. If the code keeps coming back, the transmission computer's power supply and data network connections need to be checked.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- Low battery voltage, or a temporary record left behind during a voltage drop
- A problem with the transmission computer's supply fuse, relay or ground connection
- A loose connection, corrosion, or oil or water getting into the transmission computer's connector
- Broken wiring or a short circuit in the data network (CAN) wiring
- Less often, a fault in the transmission computer (mechatronic unit) itself
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- GM
- Lost Communication With Transmission Control Module Invalid Data
Related codes
How does it appear in the 2dakika report?
Lost Communication With TCM
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.