Technical Description of OBD-II Trouble Code
Intermittent Vehicle Speed Sensor A
What is the meaning of this?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (Porsche, BMW, Ford, Cadillac, Volvo, Mini, Chevy, Chrysler, Toyota, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When the stored code P0503 is displayed, it indicates that the powertrain control module (PCM) has detected an intermittent, erratic, or excessive voltage input signal from vehicle speed sensor (VSS) A. The designation A usually refers to the primary VSS in a system that uses multiple vehicle speed sensors.
OBD II vehicle speed sensors are typically electromagnetic sensors that use a toothed reluctor ring wheel or gear, which is mechanically attached to an axle, transmission/transfer case output shaft, differential gear, or driveshaft. As the shaft spins, the metal reluctor ring also spins. The reluctor ring completes a circuit with the stationary electromagnetic sensor as the reluctor passes very close to the electromagnetic tip of the sensor. The notches between the teeth of the reluctor ring create interruptions in the sensor circuit. The PCM recognizes the combination of circuit completions and interruptions as voltage waveform patterns.
The PCM monitors vehicle speed by using input data from one or more vehicle speed sensors. It compares the inputs from the VSS with inputs from the antilock brake control module (ABCM) or electronic brake control module (EBCM). The primary VSS input (A) is usually initiated by the VSS in the transmission, but secondary VSS inputs may be monitored using one or more wheel speed sensors.
A typical vehicle speed sensor (VSS):
If the PCM detects an intermittent, erratic, or high input voltage signal from the primary VSS, it will store a code P0503 and may illuminate the malfunction indicator lamp. An intermittent, erratic, or high voltage input condition may be caused by an electrical or mechanical problem.
Related trouble codes for vehicle speed sensors:
Severity of the Code & Symptoms
Since conditions that may result in a stored code P0503 could cause drivability and ABS issues, it should be considered severe and addressed with some urgency.
Symptoms of a P0503 code may include:
Causes
Possible causes for this code include:
Diagnostic and Repair Procedures
To diagnose a code P0503, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), possibly an oscilloscope, and a reliable source of vehicle information. An ideal diagnostic tool would be a scanner with an integrated DVOM and oscilloscope.
I recommend starting the diagnosis by visually inspecting the system wiring, speed sensors, and connectors. Repair any open or shorted circuits as necessary and clean excessive metallic debris from affected sensors. If possible, also check the integrity of the entire reluctor ring when removing the sensor.
Next, connect the scanner to the vehicle’s diagnostic port and retrieve all stored trouble codes and freeze-frame data. Write down this information as it may be helpful during the diagnosis. Clear the codes and test drive the vehicle to see if the symptoms persist or if the code is reset.
A useful technique used by many professional technicians is to search the vehicle information source for relevant technical service bulletins (TSB). If you find a TSB that matches the symptoms and stored codes of the vehicle in question, the diagnostic information contained in the TSB can help in correctly diagnosing the P0503.
While test driving the vehicle, observe the wheel speed and/or vehicle speed using the scanner data stream. By narrowing down the data stream to display only relevant fields, you can increase the speed and accuracy of obtaining the desired data. Inconsistent, erratic, or high readings from the VSS or wheel speed sensors may indicate wiring, electrical connector, or sensor problems, narrowing down the general area of the system malfunction.
After pinpointing the problematic area, use the DVOM to perform a resistance test on the sensor in question. Consult your vehicle information source for the manufacturer’s recommended testing procedure for the VSS and replace sensors that do not meet the specifications. The oscilloscope can be used to retrieve live data from each individual VSS by probing the sensor signal wire and the sensor ground wire. To safely perform this type of testing, the driveline will need to be running, so securely jack or hoist the vehicle.
Vehicle speed sensors are often damaged during regular transmission maintenance, and wheel speed sensors (along with sensor wiring harnesses) are frequently broken during brake repairs. If a code P0503 is displayed immediately after a repair, suspect a damaged sensor harness or sensor.
Additional diagnostic notes:
