What does it mean?
OBD II fault code P0502 is a generic code that is defined as “Vehicle speed sensor (VSS) -low input”, and is set when the PCM (Powertrain Control Module) detects a lower than expected input signal from the Vehicle Speed Sensor (VVS). Note that on applications that use multiple vehicle speed sensors, code P0502 typically refers to the primary vehicle speed sensor, which is almost invariably located on or near the transmission’s output shaft.
In these cases, the definition for code P0502 may include the letter “A” to refer to the primary vehicle speed sensor.
In terms of operation, the PCM uses input data from the vehicle speed sensor (which it compares to input data from the ABS wheel speed sensors or electronic brake system control module) to regulate and control transmission shift patterns, as well as safety systems such as cruise -, stability -, traction control, and other systems that rely on the accurate input data from the vehicle speed sensor to operate effectively.
Bear in mind that the primary vehicle speed sensor input data will derive from the primary vehicle speed sensor located in or on the transmission. Almost all vehicle speed sensors consist of a toothed gear that engages with a drive gear on the transmission output shaft. As the outpus shaft rotates, the driven gear in the sensor drives a reluctor wheel in the sensor.
The teeth in the reluctor wheel interrupt a magnetic field as they pass in front of either a permanent or electromagnet, with the interruptions and circuit completions being interpreted by the PCM and other control modules as a wave form that represents an electrical voltage. This voltage is referred to as the “signal” voltage, which changes as the frequency of the wave generated by the sensor changes with speed.
Since the speed at which the vehicle speed sensor rotates serves as the basis from which the PCM calculates the vehicle’s actual road speed, a failure or malfunction in the speed sensor’s circuit that causes the PCM to receive a lower than expected input signal can cause a mismatch between input data from the vehicle speed sensor, and input data supplied by secondary speed sensors. If this happens, the PCM will set code P0502, and may illuminate a warning light.
Note though that this code can be caused both by circuit failures/malfunctions, and/or mechanical failure of the vehicle speed sensor itself. The image below shows a typical primary vehicle speed sensor, such as might be found fitted to the transmissions of most applications. Note however that while most vehicle speed sensors appear to be similar, almost all vehicle speed sensors are unique to their applications, and are therefore generally not interchangeable.