What does it mean?
OBD II fault code P0503 is a generic code that is defined as “Vehicle speed sensor (VSS) -intermittent/erratic/high input”, and is set when the PCM Powertrain Control Module) receives a signal from the Vehicle Speed Sensor that falls outside of the expected value.
Note that signals that fall outside of the expected range include signals that indicate both higher and lower than expected values, as well as signals that are erratic, implausible, intermittent, and/or signals that do not agree with input data received from one or more wheel speed sensors.
While the primary function of the vehicle speed sensor is to supply input data to the odometer/speedometer to indicate the vehicle’s actual road speed to the driver, this input data is also used by various other control modules and systems.
For instance, adaptive power steering systems on some applications use this data to decrease steering assistance as the road speed increases and vice versa , while the ABS control module uses the data for comparison purposes during the calculation of an appropriate braking strategy.
Other applications use this input data to control the applications’ ride height and suspension settings, while still other (high-end) applications use the data from the vehicle speed sensor to activate and control features in/on the bodywork to improve aerodynamics, and increase down force.
Moreover, mundane systems such as cruise control, transmission-shifting strategies, and engine driveability settings are all affected by the vehicles’ actual speed, meaning that if the PCM receives inaccurate input data from the vehicle speed sensor, almost every aspect of the vehicles’ driveability and/or efficient operation can be influenced negatively.
In practice, most applications use only one speed sensor that is usually located near the transmission’s output shaft, but be aware that some applications use more than one speed sensor in different locations. In some cases, one or more wheel speed sensors do double duty as vehicle speed sensors, but this is relatively rare, and applies mostly to high-end, or supercar applications that employ various types of all-wheel-drive systems.
Nonetheless, most vehicle speed sensors are of the electromagnetic type that works in conjunction with a reluctor ring. As the teeth of the reluctor ring pass in front of sensor, changes are induced in the sensors’ magnetic field, which changes are interpreted by the PCM as changes in voltage. These voltages are in their turn then processed, and finally “translated” by the PCM into a value that corresponds to a road speed, which the driver can read off the speedometer as miles (or km) per hour.
Note however, that in many cases, the speed-sensing element in the sensor is driven by a gear that meshes with another gear in the transmission. In these cases, there is no reluctor ring involved.
From the above, it should be obvious that incorrect, invalid, implausible, or erratic input data from the vehicle sped sensor can have serious effects on the driveability of a vehicle, which is why the PCM will set code P0503, and illuminate a warning light if it detects a deviation that can be as small as 1-2 miles per hour between the road speed as indicated by the vehicle speed sensor, and the road speed the PCM expects to see given the operating conditions that obtain at any given moment.
Note that although the length of time that needs to elapse from when the fault is first detected, to the moment the code is stored varies between applications, the time required is usually between 3 and 5 consecutive seconds. The image below shows a gear-driven vehicle speed sensor (top), and a magnetic vehicle speed sensor (bottom) that works in conjunction with a suitably located reluctor ring.