DTC P0500: VSS Circuit: Description
WARNING: This page is about a different car, the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, and 2002 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
The vehicle speed output signal is generated by the Powertrain Control Module (PCM). The signal is a duplicate of the Vehicle Speed Sensor (VSS) signal that the PCM receives from the VSS. The PCM contains an integrated circuit called a driver that switches the signal circuit to ground at a rate of 4000 pulses for every mph the vehicle is driven. Various modules (IP, Cruise, AF ECM, etc.) that are connected to the vehicle speed output circuit convert the 4K PPM into a vehicle speed value.
This DTC is used to indicate that the AF ECM cannot monitor an accurate vehicle speed.