When the DTC P0503 OBDII code is set, it is because the Powertrain Control Module (PCM) has detected a voltage input signal from the Vehicle Speed Sensor (VSS), which is erratic or intermittent.
The PCM monitors the vehicle speed using data from one or more speed sensors. It compares the VSS sensor inputs with the Anti-lock Braking System (ABS) inputs.
If the PCM detects an intermittent input voltage signal from the primary VSS Sensor, this code will be stored and the malfunction light will illuminate.