U0121 Lost communication with the ABS control module
- System
- Network - modules not talking to each other
What U0121 means
Modules on the car's shared network send each other regular messages. The name "Lost communication with the ABS control module" says the ones carrying that module's data stopped arriving at the module expecting them, so the measurement is the presence of a message on the network, not a brake or wheel figure. It names the module rather than the brakes, which is why it is a U code and not one of the C0xxx chassis codes.
This is a standard code. It means the same thing on every make of vehicle.
What the program shows for U0121
Nothing on the program's list of readings comes from the module this code names: the nearest is the Dashboard's single Vehicle speed gauge, the engine computer's own figure rather than a speed per wheel, and the Trip screen's Distance and Average speed are worked out from that same figure. The names the Full scan can put on the On-board self-tests it reads run from Oxygen sensor and Catalyst bank through EGR / VVT bank, EVAP purge valve and Misfire cylinder, with nothing from the ABS side among them.
The Trouble Codes screen names the module the car lost contact with, from the engine and transmission modules to ABS, body, restraints and the instrument cluster, where the module is on its list. Look up a code does the same offline and says whether a code is standard or manufacturer-specific, and the Adapter traffic screen shows the raw requests and replies. Compare between two recordings shows whether the code is Gone, New or Still there.
See how the program shows a code →