U0100 and U0101: lost communication with the engine or transmission module
A module on the car's network stopped hearing from the engine computer (U0100) or the transmission module (U0101).
SeverityHighCarrying on can cause expensive damage, so deal with it now.
Can you keep driving?Only as far as the workshopWhat does U0100 mean?
The car's modules talk to each other over a network, usually CAN, and each one expects regular messages from the others. When the engine or transmission module goes silent for longer than allowed, the modules that miss it store a U code. The silent module may have lost power, its network wiring may be damaged, or the module itself may have failed.
What causes U0100?
- A weak battery, or a voltage drop while cranking
- A blown fuse or a failed power relay for the module
- Damaged network wiring or a corroded connector
- Water getting into a module or connector
- A failed module
What to check first for U0100
- Check the battery and its terminals first: low voltage causes many network codes
- Check the fuses and relays that supply the silent module
- Look for water or corrosion at the module connectors
- Have the network checked with a scan tool that shows which modules answer
Can you drive with U0100?
If the engine computer is the module that went silent, the engine may cut out, refuse to start or run in a protective mode. Drive to a workshop only if the car runs normally, and have it towed if it stalls or shows several warning lights at once.
In states with an OBD-II emissions inspection, a car with the check engine light on fails the test until the fault is fixed.
Codes on this page
- U0100Lost Communication With ECM/PCM "A"
- U0101Lost Communication with TCM
Related codes
Checked on · Rafael Soares
These are the generic meanings every manufacturer shares. Each brand adds its own detail, so your car's handbook, its service information and a qualified mechanic have the last word. OdoKeep has no connection with any manufacturer, and the app does not read codes from the car.
