What does it mean?
OBD II fault code U0415 is a generic code that is defined as “Invalid Data Received from Anti-Lock Brake System (ABS) Control Module”, and is set when the PCM (Powertrain Control Module) detects or receives a signal from the ABS control module that is invalid, implausible, erratic, or that otherwise falls outside of the expected range. All applications use a dedicated control module to monitor and control the operation of the ABS (Anti-lock brake) system.
In terms of operating principles, the ABS control module receives input from individual wheel speed sensors, whose function it is to measure the rotational speed of each individual wheel. In practice, and on a fully functional ABS brake system, the ABS control module compares the rotational sped of each wheel many times per second, and on a vehicle that is traveling in a straight line, the wheels will rotate at the same speed.
This information is also relayed to the PCM, which in turn, shares this input data with other systems, such as the traction-, and stability control modules, among others. If the brakes are applied, the wheels should (in theory) slow down at the same rate.
However, in cases where the wheels do not slow down at the same rate, such as might happen when say, two wheels on one side of the vehicle are on the road surface and two wheels are off the road surface, the wheels that are off the road surface might lock up under braking.
When this happens, the locking wheels will rotate at a much slower rate than the wheels that are on the road surface, and as a result, the ABS control module will partially release the braking force on those wheels until all the wheels again rotate at the same rate. This process can occur several times per second, and the practical advantage of this happening is that the driver retains full steering control of the vehicle, even under emergency braking conditions.
Moreover, based on input data from other sensors such as the vehicle speed sensor, steering angle sensor, throttle position sensor, and one or more yaw sensors, both the ABS and Powertrain Control Modules may recognize a dangerous situation, and apply differential braking forces via the ABS brake system selectively to individual wheels in order to stabilize the vehicle.
From the above it should obvious that the PCM requires valid data from the ABS system at all times via the wheel speed sensors in order to be able to control the brake -, and other brake-dependent system in a safe and effective manner. Thus, if the PCM detects input data from the ABS that is implausible, or invalid in any way, it will set code U0415, and also illuminate a warning light.
Note that on most applications, the PCM will also disable or deactivate the ABS system until the fault is corrected.