OBD-II Trouble Code Technical Description
Low Input Detected in Vehicle Speed Sensor A
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (Chevy, Chevrolet, Isuzu, Cadillac, Subaru, Saab, Volvo, GMC, GM, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
A stored code P0502 indicates that the powertrain control module (PCM) has detected a low voltage input signal from vehicle speed sensor (VSS) A. The designation A typically refers to the primary VSS in systems that use multiple vehicle speed sensors.
Most vehicle speed sensors are electromagnetic sensors that use a toothed reluctor ring wheel or gear permanently attached to an axle, transmission/transfer case output shaft, differential gear, or driveshaft. As the shaft spins, the reluctor device also spins, completing a circuit with the stationary electromagnetic sensor. The notches in the reluctor ring create interruptions in the sensor circuit as they pass very close to the electromagnetic tip of the sensor. The PCM (and other controllers) receive these circuit completions and interruptions as waveform patterns representing voltage.
The PCM monitors vehicle speed by using input data from one or more vehicle speed sensors and comparing it to inputs from the antilock brake control module (ABCM) or electronic brake control module (EBCM). In some cases, secondary VSS input may be monitored using one or more wheel speed sensors, but the primary VSS input (A) is usually initiated by the VSS in the transmission.
If the PCM detects a low input voltage signal from the primary VSS, it will store a code P0502 and may illuminate a malfunction indicator lamp. A low voltage input condition can be caused by either an electrical or mechanical problem.
A typical VSS or vehicle speed sensor:
Related vehicle speed sensor trouble codes:
Code Severity & Symptoms
Since the conditions that may cause a P0502 code to be stored could lead to drivability and ABS issues, it should be considered severe and addressed as soon as possible.
Symptoms of a P0502 code may include:
Causes
Possible causes for this code include:
Diagnostic and Repair Procedures
When diagnosing a code P0502, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), possibly an oscilloscope, and a reliable vehicle information source.
It is recommended to start the diagnosis of a P0502 with a visual inspection of the system wiring, speed sensors, and connectors. Repair any open or shorted circuits as necessary and clean excessive metallic debris from affected sensors. When inspecting the sensor, check the integrity of the reluctor ring.
Connect the scanner to the vehicle diagnostic port and retrieve all stored trouble codes and available freeze-frame data. Before clearing the codes, make a note of this information as it may be helpful during the diagnosis process.
Use your vehicle information source to search for applicable 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 assist in diagnosing the P0502.
Utilize the scanner data stream to observe wheel speed and vehicle speed while test driving the vehicle. You can narrow the data stream to display only relevant fields to increase the speed and accuracy of the desired data. Inconsistent or erratic readings from VSS or wheel speed sensors may indicate wiring, electrical connector, or sensor problems, helping to narrow down the general area of the system malfunction.
Once you have identified the VSS circuit from which the low voltage input is being initiated, 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 specifications.
Use the oscilloscope to retrieve live data from each individual VSS by probing the sensor signal wire and the sensor ground wire. Jacking or hoisting the vehicle will usually be necessary to perform this type of testing properly. After securing the drive axle/s off the ground, run up the drivetrain while observing the waveform pattern on the oscilloscope. Pay attention to any glitches or inconsistencies in the pattern and proceed with the diagnosis accordingly.
Vehicle speed sensors may be damaged as a result of regular maintenance, while wheel speed sensors and sensor wiring harnesses are often damaged during brake repairs. If this code appears immediately after a repair, suspect a damaged sensor harness, connector, or sensor.
Additional diagnostic notes:
