DTC P0502: Vehicle Speed Sensor Circuit - Low Output: Description

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 Pontiac Grand Prix, 2002 Pontiac Bonneville, 2002 Chevrolet Monte Carlo, 2002 Chevrolet Impala, and 2002 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. This information is then sent to the PCM to indicate how fast the vehicle is traveling and to develop the shift pattern for the transmission. Other systems also use VSS information:

TCC apply and release.
Cruise control system.
Fuel delivery systems.

DTC sets when PCM detects a low vehicle speed with a high engine speed while in a drive range.