What does it mean?
OBD II fault code U0123 is defined as “Lost Communication with Yaw Rate Sensor Module”, or sometimes as “Lost Communication with Yaw Rate Sensor”, and is set when a loss of communication occurs between the Yaw Rate Sensor/Module, and the other control modules that control the ABS, traction control, and stability control systems on a modern vehicle. “Communication” refers to the constant exchange of data between various controllers via the CAN bus (Controller Area Network) system.
When the loss of communication exceeds 1 second on most applications, a code will be set and warning lights such as the ABS, Stability Control, and Traction Control will be illuminated. Note that Stability Control and/or Traction Control warning lights may flash. In terms of its operation, the yaw rate sensor/module measures the difference between the intended path, and the actual path the vehicle takes during cornering or violent evasive manoeuvres to avoid obstacles.
However, the yaw rate sensor/module and its associated control modules form only one part of the equation that determines, or controls vehicle stability; other information relevant to the equation includes vehicle road speed, throttle position, transmission mode, and in some cases, even the proximity of other vehicles and/or obstacles.
Nonetheless, the yaw rate sensor/module is typically inactive when the vehicle is moving in a straight line, and only becomes active when the steering angle changes. The amount of steering input is sent to the yaw rate sensor/module, where this information is compared to the road speed, throttle position and other parameters, such as data on lateral accelerations obtained from an accelerometer.
If the yaw rate sensor/module determines that the vehicle is deviating from the intended path (based on the steering angle, road speed, and individual wheel speeds), and that the deviation is likely to cause a dangerous situation such as a skid, it will activate the stability control system to apply selective braking forces to one or more wheels via the ABS system to force the vehicle to follow the intended steering path.
Note that in some cases, the stability control system may assume temporary control of both the throttle and transmission until the various systems involved in maintaining vehicle stability deem the vehicle to be under control, or to be following the intended steering angle. The image below illustrates the types of deviations from the intended steering angle the yaw rate sensor/module is intended to identify and correct.
In both under -, and over steer conditions, the yaw rate sensor/module activates the stability control system to force the vehicle to follow the intended steering angle as communicated by the steering angle sensor.