U0101
Lost Communication With TCM
Communication network
Generic OBD-II
High
What it means
U0101 means the ECM has stopped receiving data from the transmission control module (TCM) over the vehicle's communication (CAN) bus. Without TCM data the transmission usually drops into a fail-safe "limp" mode — stuck in one gear with harsh or no shifting — and warning lights come on. The engine itself normally still starts and runs. Damaged bus wiring, bad grounds, or a failed TCM are the usual causes.
High
Can I keep driving? Limited — the transmission may drop into limp mode (one gear, harsh shifts); drive gently to a shop and avoid highway speeds.
Common symptoms
What you'll notice
- Check-engine light plus a transmission warning light
- Transmission stuck in a single fail-safe gear
- Harsh shifting, or no shifting at all
- Speedometer or gauge dropouts
- A scan tool can't communicate with the TCM
Common causes
Most likely first
- Damaged or corroded CAN bus wiring or connectors at the TCMLikely
- Poor TCM power or ground, or low battery voltageCommon
- Failed transmission control module (TCM)Common
- A bus fault dragging down communicationLess often
- Rarely, a software or ECM issueLess often
How to diagnose & fix
Work through it in order
- Check battery voltage and the TCM's power and ground connections first.
- Scan all modules and note which ones are unreachable to narrow it down.
- Inspect the CAN bus wiring and connectors at the TCM for damage or corrosion.
- Back-probe the bus circuits for opens or shorts.
- Repair the wiring, or reprogram/replace the TCM; clear and re-test.
FAQ
Quick answers
Is it safe to drive with U0101?+
Only briefly and gently. With no TCM data the transmission usually locks into a fail-safe gear, so avoid highway speeds and get it to a shop. The engine itself normally still runs.
Can I just clear U0101?+
It returns as soon as communication drops again. The wiring, connection, or TCM behind the lost link has to be repaired.
Related codes
Often seen with U0101
Read & clear U0101 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX