Code C0504 indicates that the ABS control module has detected an erratic or intermittent signal from the left front wheel speed sensor. This means the signal is not completely lost, but it is fluctuating outside of the expected parameters for the vehicle's current speed.
The sensor internal electronics have degraded, causing inconsistent signal output.
Debris, rust, or a cracked reluctor ring prevents the sensor from reading a clean pulse.
Frayed wires, corrosion in connectors, or rubbing against chassis components causes intermittent shorts or opens.
The sensor is not seated properly, creating an improper air gap between the sensor tip and the tone ring.
Internal circuit failure within the ABS module preventing correct processing of the sensor signal.
The vehicle is drivable, but the ABS, traction control, and stability control systems will be disabled, which can increase braking distances on slippery surfaces.
It is unlikely. Most erratic signal codes are caused by the sensor, damaged wiring, or a dirty tone ring; the module is rarely the cause.
Intermittent signals are often caused by wiring that shorts only when the suspension moves, or by debris on the tone ring that only disrupts the signal at certain speeds.