Fault code U0101 – a step-by-step guide to diagnosing and fixing it

Error code U0101 is displayed and the check engine light is on - what does this mean?

DTC U0101 is triggered by the OBD/EOBD system when communication between the engine control unit (ECM/PCM) and the transmission control unit (TCM) is lost or interrupted. This error code indicates a problem in the CAN bus network, which enables data transmission between the various control units.

What can cause this error code?

• Defective transmission control unit (TCM)
• Damaged CAN bus wiring between ECM and TCM
• Corroded or loose plug connections
• Defective engine control unit (ECM/PCM)
• Interrupted or short-circuited data lines
• Voltage problems in the vehicle electrics
• Defective CAN bus terminating resistors
• Software errors in the control units

What symptoms occur in parallel with fault code U0101?

• Shifting problems: The automatic transmission shifts jerkily or gets stuck in a gear
• Emergency running program: The transmission switches to safety mode with limited functionality
• Starting problems: The engine is difficult to start or stalls again after starting
• Warning lights: In addition to the engine warning light, transmission or ABS warning lights may also come on
• Loss of power: Reduced engine power and poor acceleration
• Irregular idling: The engine runs erratically or stutters when idling

How to fix the problem of error code U0101?

• The repair requires a systematic diagnosis of the communication paths between the control units.
• Check and clean the plug connections: Check all electrical connections between the ECM and TCM for corrosion, looseness or damage and clean or replace if necessary
• Test CAN bus cabling: Check the data bus cables for continuity, short circuits and correct resistance values using a multimeter and replace damaged cables
• Replace the transmission control unit: If the TCM is defective, replace the control unit with a new or overhauled one and then have it programmed
• Check the engine control unit: Test the ECM/PCM for functionality and replace and reprogram if defective
• Carry out a software update: Install the latest firmware updates for both control units to rectify known communication errors
• Check the power supply: Measure the power supply to both control units and repair the corresponding fuses, relays or wiring in the event of deviations