P0502 – Vehicle speed sensor (VSS) -low input
Last Updated 2017-06-19
Automobile Repair Shop Owner
|Code||Fault Location||Probable Cause|
|P0502|| Vehicle speed sensor (VSS) -low input |
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|Wiring short to earth, VSS, ECM|
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Table of Contents
- What Does Code P0502 Mean?
- What are the common causes of code P0502 ?
- What are the symptoms of code P0502 ?
- How do you troubleshoot code P0502 ?
- Codes Related to P0502
- Get Help with P0502
What Does Code P0502 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.
What are the common causes of code P0502 ?
Some common causes of code P0502 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors
- Defective speed sensors, including damage to reluctor rings
- Excessive metal wear particles or other debris that affect the proper operation of speed sensors
- Failed or failing PCM or other relevant control module(s). Note that control module failure is rare, and the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code P0502 ?
WARNING: While the possible symptoms of code P0502 vary between applications, this code should NOT be ignored, since some symptoms can appear unexpectedly and have serious negative impacts on driveability and safety. Some possible symptoms of code P0502 are listed below-
- Stored trouble code and possibly, one or more illuminated warning lights, which could include the ABS light
- Erratic or incorrect operation of the speedometer/odometer will be present on most applications
- Gear shifts may be harsh, erratic, or unpredictable on some applications
- One or more additional codes may be present, including codes relating to the transmission, cruise control, traction control, ABS system, and stability control systems
- On some applications, unexpected and/or erratic activation/deactivation of one or more safety systems may occur
- On some applications, the ABS system may be deactivated automatically, and will remain deactivated until the code is resolved
How do you troubleshoot code P0502 ?
NOTE #1: Apart from a digital multimeter and repair manual/wiring diagram for the application, an oscilloscope may be required to diagnose some sensor failures. Thus, if the code persists despite several repair attempts and an oscilloscope is not available, the wiser option would be to refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair.
NOTE #2: On many applications, it is common for speed sensors to be damaged during routine repairs/maintenance, especially when performing brake repairs or major repairs in the engine compartment. Thus, if code P0502 appears immediately after a repair, suspect a damaged speed sensor on the transmission, or damaged sensors, wiring, and/or connectors near the brakes if brake repairs had been performed.
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: If other codes are present along with P0502, take careful note of the order in which they were stored. It is common for ABS, stability control, traction control, and transmission codes to accompany P0502, so refer to the manual for details on the relationships between P0502 and any other codes that may also be present, but note that codes relating to the systems above that follow P0502 will almost certainly have set as a result of P0502. Bear in mind that codes that precede P0502 must also be investigated and resolved before an attempt is made to diagnose P0502 if the manual call for these codes to be resolved first. Failure to do this will almost certainly result in a misdiagnosis, wasted time, and the unnecessary replacement of parts and components.
Assuming that there are no other codes present, or that additional codes have been resolved, refer to the manual to identify and locate the primary vehicle speed sensor on the transmission, as well as all associated wiring and connectors. Be sure to determine the function, color-coding, and exact routing of all relevant wiring at this point to avoid testing the wrong circuit later on.
Perform a thorough visual inspection of all wiring and connectors. Look for damaged, burnt, shorted, disconnected, and corroded wiring and or connectors. Make repairs, or replace wiring as required.
Clear all codes after repairs are complete, and operate the vehicle normally before rescanning the system to see if any codes return.
If the fault persists but there is no visible damage to the wiring, prepare to perform resistance, continuity, ground, and reference voltage tests on all associated wiring, but be sure to disconnect the vehicle speed sensor from all relevant control modules to prevent damaging one or more modules during this step.
If the sensor requires a 5-volt reference voltage (refer to the manual to determine this), pay particular attention to this circuit. Check this wire’s resistance and continuity between the sensor connector and the CORRECT pin on the PCM connector. If these values check out, refer to the manual on the correct procedure to follow to test whether the PCM is actually delivering a 5-volt current at this connector. If it does not, replace the PCM. Note however that it is rare for a PCM to fail to deliver the correct reference voltage.
If the reference voltage checks out, compare all other obtained readings with the values stated in the manual, and make repairs or replace wiring as required if deviations from stated values are found. Clear all codes after repairs are complete, and operate the vehicle normally before rescanning the system to see if any codes return.
NOTE: Take note that the vehicle speed sensor itself forms part of its control circuit, and as such, its internal resistance must be tested as well. As with many other sensors, its internal resistance is a reasonably good indicator of its overall condition, so replace the sensor if its resistance does not agree with the specified value.
If all electrical values check out, operate the vehicle normally with the scanner connected so that an assistant can monitor the operation of the vehicle speed sensor in real time. However, narrow the scanners monitoring function down to only include data from the speed sensor in order to obtain the most accurate test results.
On most scanners, the data from the speed sensor will be displayed as numerical values, usually expressed as a speed in m/ph, or as a voltage, or sometimes as both on the same screen. Operate the vehicle at a steady speed and compare the data from the scanner with the reading on the speedometer. However, bear in mind that almost all speedometers over-, or under indicate the vehicles’ actual speed; determine the degree of error before testing the vehicle, and only condemn the vehicle speed sensor if the deviation between the scanner data and the speedometer exceeds this value.
Nonetheless, the object of this test is to see if the vehicle speed sensor delivers a constant voltage/reading at a steady road speed. Thus, keeping official speed limits/restrictions in mind operate the vehicle at a wide range of road speeds; if the scanner input data deviates from the indicated speed by more than the allowable amount at any speed, or if the displayed data fluctuates at a steady speed, the vehicle speed sensor is defective, and it must be replaced.
NOTE: Be aware that on some applications, removing the vehicle speed sensor will allow some transmission fluid or lubricant to escape. If you are not comfortable with the idea of dealing with hot transmission fluid, or if the transmission has a so-called “lifetime” fill, refer the vehicle to the dealer or other competent repair facility for professional assistance with the sensor replacement.
Steps 1 through 6 will almost certainly resolve code P0502 on most applications; however, if the affected application uses more than one speed sensor, perform the all of the steps outlined above on all speed sensors until the fault is both identified and repaired or until it becomes clear that these steps cannot resolve the issue.
Since it is likely that the fault might involve the incredibly complex CAN (Controller Area Network) bus system on these applications, an oscilloscope and wave form library may be required to diagnose the fault. If an oscilloscope and relevant reference data are not available, the better option would be to refer the vehicle to the dealer or other competent repair facility for professional diagnoses and repair, since cheap, generic code readers can usually not access the CAN bus system.
Codes Related to P0502
- P0500 – Relates to “Vehicle Speed Sensor “A” Malfunction”
- P0501 – Relates to “Vehicle Speed Sensor “A” Range/Performance”
- P0503 – Relates to “Vehicle Speed Sensor “A” Intermittent/Erratic/High”
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