DTC P0503: Vehicle Speed Sensor Circuit - Erratic Output: Description
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
Vehicle Speed Sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in case extension, and a toothed vehicle speed sensor reluctor wheel, which is pressed onto final drive carrier assembly. As vehicle drives forward, vehicle speed sensor reluctor wheel rotates. This rotation produces a variable AC signal in pick-up coil that is proportional to vehicle speed. PCM uses this signal to calculate vehicle speed, shift timing and gear ratios. DTC sets when PCM detects a low vehicle speed with a high engine speed while in a drive range. DTC sets when PCM detects a large change in vehicle speed in a short period of time.