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Error Code P0502 is defined as Vehicle Speed Sensor Low Input. This means the vehicle’s computer has detected a different reading of speed between the speed sensor of the vehicle and the wheel speed sensors.
This is a generic trouble code, which means it applies to all vehicles made from 1996 up to the present, particularly those equipped with the OBD-II system like in Cadillac, Chevrolet, GM, GMC, Saab, Subaru, or Volvo. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
Most vehicles come equipped with electromagnetic speed sensors. This type of sensor uses a toothed reluctor ring wheel or gear affixed to an axle, transmission/transfer case output shaft, driver shaft, or differential gear. The reluctor device spins with the shaft pins, completing a circuit with the stationary electromagnetic sensor. When the reulactor passes in very close proximity to the electromagnetic tip of the sensor, the notches in the reluctor ring makes interruptions to the sensor circuit. The combinations of these circuit completions, as well as other interruptions, are received by the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) and other controllers, as waveform patterns that represent voltage.
Data from one or more vehicle speed sensors are used by the PCM to monitor the speed of the vehicle, and then compare it to inputs from the ABCM (antilock brake control module) or EBCM (electronic brake control module). In some vehicle makes and models, a secondary VSS input may be monitored using one or more wheel speed sensors, but the primary VSS input (A) will likely be initiated by the transmission’s VSS.
When the PCM starts to see a low input voltage from the primary VSS, it will activate the Check Engine light and register the Error Code P0500. This problem could be caused by either electrical or mechanical problem.
Other related error codes include:
- Error Code P0500 Vehicle Speed Sensor “A” Malfunction
- Error Code P0501 Vehicle Speed Sensor “A” Range/Performance
- Error Code P0503 Vehicle Speed Sensor “A” Intermittent/Erratic/High
As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. It also activates the vehicle’s antilock brake lamp or traction control lamp. Other common symptoms would include:
- Unanticipated activation and deactivation of the traction control system (if vehicle is equipped)
- Irregular transmission shift patterns
- Erratic operation of speedometer or odometer
- ABS is inoperable (in some cases)
There may also be other transmission codes and ABS codes present.
Common causes for this code include:
- Excessive buildup of metal debris on speed sensor(s)
- Faulty vehicle speed sensor
- Faulty wheel speed sensor
- Cuts or damages in the connectors or wiring harness (especially near the speed sensors)
- Fault or worn teeth on reluctor ring
- Defective PCM, ABCM, or EBCM
How to Check
To diagnose this code, you will need a DVOM (digital volt-ohmmeter), oscilloscope, and a vehicle information source.
Start your diagnosis through a visual inspection of the wiring system, connectors, and speed sensors. Repair open or shorted circuits as needed, and clean excessive metallic debris from affected sensors. Also, check the integrity of the reluctor ring when inspecting the sensors.
Retrieve all stored trouble codes and their freeze-frame data by connecting the scanner to the diagnostic port of the vehicle. Write all of the information down and then clear the codes.
Use your vehicle information source to search for the applicable TSB (technical service bulletins). Use the symptoms of your vehicle to match the information.
Observe both wheel speed and vehicle speed using the scanner data during your test drive. You can narrow the data stream to show only the pertinent fields, and to increase the speed and accuracy where the data is delivered. When you see erratic or inconsistent readings from the VSS or wheel speed sensors, then the problem may be in the wiring, electrical connector, or sensors.
Find the VSS circuit from which low voltage input comes from, then perform a resistance test on the sensor in question using the DVOM. Refer to your vehicle information source for the specific recommendations from the manufacturer. Sensors that don’t comply with the specifications must be replaced.
Retrieve data from each individual VSS using the oscilloscope by probing the sensor signal wire and the sensor ground wire. You may have to jack or hoist the vehicle to perform this test properly. After the drive axle/s are safely secured off the ground, run up the drivetrain while observing the waveform pattern on the oscilloscope. Take note of the glitches or inconsistencies in the pattern and proceed with the diagnosis accordingly.
Brake repairs can result in damage to the wheel speed sensors and sensor wiring harnesses, while vehicle speed sensors can be damaged by regular maintenance. If this code shows up right after a repair, then suspect a damaged sensor, connector, or harness.
How to Fix
- Repair or replacement of open or shorted circuits in the speed sensors
- Repair or replace damaged wiring or connector
- Clean excessive metallic debris from affected sensors
- Replacement of vehicle speed sensor
- Repair or replacement of wiring or connectors in the vehicle speed sensor circuit
- Replacement of one or more wheel speed sensors
- Replacement of a hub bearing
- Replacement of CV axle due to a faulty tone ring
- Replacement of instrument cluster
- Fitting the correct size wheels and tires to the vehicle
- Replacement of PCM (rare)
This trouble may not exhibit drivability problems, but an inaccurate speedometer can result in a speeding ticket.