Code U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module A

U0121 indicates loss of contact with a chassis-dynamics module -- ABS/stability control, steering angle sensor, ride-height/suspension control, power steering, or a differential/brake-blending actuator -- all mounted low near the subframe, wheel wells, or under-floor tunnel where connectors take the worst exposure to road salt, water spray, and vibration of any module class. Unlike cabin electronics, these connectors are often sealed but still fail at the seal itself after years of thermal cycling, so a corroded or water-intruded low-mount connector is the dominant cause ahead of any harness break.