DTC P0503: Vehicle Speed Sensor Erratic: Description
WARNING: This page is about a different car, the 2002 Pontiac Montana, 2002 Oldsmobile Silhouette, and 2002 Chevrolet Venture. However, it is still accessible from the selected car via links, so may be relevant.
The Vehicle Speed Sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case extension, and a toothed vehicle speed sensor reluctor wheel, which is pressed onto the final drive carrier assembly. As the vehicle drives forward, the vehicle speed sensor reluctor wheel rotates. This rotation produces a variable AC signal in the pick-up coil that is proportional to vehicle speed. The PCM uses this signal to calculate vehicle speed, shift timing and gear ratios. If the PCM detects a large change in vehicle speed in a short period of time, then DTC P0503 sets.