Audi A4 allroad ABS Light is on Inspection at your home or office.

Common reasons for this to happen:

Faulty Wheel Speed Sensor

The ABS relies on information from the wheel speed sensors, which are placed inside the hub of each wheel and occasionally inside the transmission. These sensors determine the rate at which the wheels are moving, and relay that information to the ABS. When the ABS receives information that one or more of the vehicle’s wheels are moving slower than the others, the system releases brake fluid pressure, which then allows the wheels to resume movement. If the wheel speed sensor is defective, or if the wiring from the sensor to the ABS is damaged, then the ABS will not get the wheel speed information that it requires.

The wheel speed sensor is made up of two components: a metal ring with teeth attached to a drive axle or wheel bearing, and the sensor itself, which reads the teeth on the ring as the wheel rotates. As the wheel speed sensor is located very close to the braking system, the high heat generated by the brakes cause the wheel speed sensor to be more susceptible to damage. This in turn makes it one of the more common causes of the ABS light coming on.

Unresponsive Hydraulic Pump or Valve

When the wheel speed sensor alerts the ABS that the wheels are moving at different speeds, a hydraulic valve is responsible for releasing, or increasing, the brake fluid pressure to prevent the wheels from losing traction with the ground. Like all pumps and valves, those connected to the ABS are liable to become damaged and unresponsive. This may be due to internal wear typically from dirty brake fluid.

Faulty ABS Module

Corrosion on the ABS module can keep information from being relayed between the wheel speed sensor and the ABS. Corrosion is a relatively common issue, and one that can cause problems in the aforementioned wheel speed sensor wires. Even a small amount of corrosion can cause enough resistance on the wires to keep the system from working. In less common instances, the computer in the module can fail and need to be replaced.

Low Fluid Levels

The ABS relies on fluid to help regulate pressure. If the ABS fluid reservoir is low, or if there is too much air in the system, then the ABS will not be able to do its job.