|Trouble Code||Fault Location||Probable Cause|
|P0503||Vehicle speed sensor (VSS) -intermittent/erratic/high input||Wiring, poor connection, other connected system, instrument panel, VSS|
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What Does Code P0503 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.
What are the common causes of code P0503 ?
Common causes of code P0503 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors
- Defective vehicle speed sensor
- Over sized or non-standard wheel/tire combinations that affect the calibration of the vehicle speed sensor circuit(s) in the PCM and other control modules
- Damaged or defective reluctor rings, especially where the reluctor ring is located in wheel bearings. In these cases, damage to the bearing often causes damage to the reluctor ring as well
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P0503 ?
Some common symptoms of code P0503 could include the following-
- Stored trouble code and an illuminated warning light
- Malfunctions in either, or both, the speedometer and odometer. Typical malfunctions include erratic readings, indicating needle that moves rapidly between extremes, or between any two points on the scale, or the instrument may not work at all
- The tachometer may behave in a similar fashion, or it may not work at all
- Cruise -, ABS -, stability -, or traction control system(s) may be deactivated
- Rev limiter may come into operation at lower than normal engine speeds
- Gear shifts on automatic transmissions may be harsh, erratic, unpredictable, or the transmission may not shift at all when the applications enters failsafe, or limp mode
How do you troubleshoot code P0503 ?
NOTE: Take note that on applications that use magnetic vehicle speed sensors and reluctor rings, it may become necessary to employ an oscilloscope to diagnose some types of issues, such as broken, damaged, or fractured reluctor rings that are located in inaccessible places- such as inside a transmission or a wheel bearing.
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: Pay particular attention to any additional codes that may be present- both active and pending. Note that ABS, and particularly wheel speed sensor related codes often contribute to the setting of P0503, which means that all such codes (and especially if such additional codes precede P0503) must be investigated and resolved before a diagnosis and repair of P0503 is attempted. Failure to do this will almost certainly result in a misdiagnosis, and the unnecessary replacement of parts and components.
Assuming that there are no additional codes present, refer to the manual to locate and identify all relevant sensors. Note that on some applications there may be more than one vehicle speed sensor, so be sure that ALL relevant sensors are properly identified before starting a diagnostic procedure for this code.
Also, use this time to determine the exact routing, color-coding, and function of each wire in the relevant control circuits to avoid testing the wrong circuit/component later.
Once all associated and relevant wiring is properly identified, perform a thorough visual inspection of all wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs, or replace wiring as required.
NOTE: One of the most common causes of this code is wiring that has rubbed through against the bodywork as a result of the wiring not being secured properly. Therefore, it is imperative that all associated wiring be thoroughly inspected for this kind of damage, since it often results in short circuits.
If no visible damage to wiring is found, prepare to perform reference voltage (if applicable), ground, resistance, and continuity tests on all wiring, but be sure to disconnect all wiring from the PCM and other controllers to prevent damage to one or more controllers during resistance and continuity checks.
Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
Note that the affected sensors form part of their control circuits, and as such, all relevant and affected sensors must be tested as well. Refer to the manual for the application for details on the correct testing procedures to follow, and replace all sensors that do not comply with the manufacturers’ specifications in any way, or to any degree.
If electrical values comply with specifications, remove the vehicle sped sensor, and inspect it for the presence of metal shavings or particles, and especially sensors that are located on the transmission.
An accumulation of metal particles on a magnetic sensor can seriously affect the operation of that sensor, but more importantly, an excessive accumulation of metal particles indicates serious mechanical trouble inside the transmission. Nonetheless, remove all metal particles from the sensor, reinstall it, clear all codes, and operate the vehicle normally to see if the code returns.
WARNING: Note that on some applications, some transmission fluid or lubricant may be lost when the vehicle speed sensor is removed. Be prepared for this, and be sure to replace all the fluid/lubricant that is lost in this way to prevent possible damage to the transmission later on.
NOTE: On sensors that are gear driven, inspect the small driven gear on the end of the sensor for any signs of damage, or abnormal wear. Note that this gear is usually made of a soft plastic material, and it does not take much damage to cause it to slip on the steel drive gear in the transmission. In fact, any wear on the driven gear should be suspected of being the cause of this code, but be sure to replace the entire sensor with a unit that has the same number of teeth on the gear as the original both to maintain the systems’ calibration, and to ensure that the sensor fits the application. Clear all codes after the sensor replacement, and operate the vehicle normally to see if the code returns.
If the code persists, but all electrical values check out, all sensors conform to the manufacturers’ specifications, and there is no damage to wiring, suspect either a faulty PCM or a defective reluctor ring where a reluctor ring is used in conjunction with the vehicle speed sensor.
However, PCM failure is rare, which means that a defective reluctor ring is the more likely cause, but in many cases, the reluctor ring is not accessible, or gaining access to it may require the disassembly and/or of unrelated parts. If a defective reluctor ring is suspected, the wiser option would be to refer the vehicle to the dealer, or other competent repair shop for professional diagnosis and repair.
Codes Related to P0503
- P0500 – Relates to “Vehicle Speed Sensor “A” Malfunction”
- P0501 – Relates to “Vehicle Speed Sensor “A” Range/Performance”
- P0502 – Relates to “Vehicle Speed Sensor “A” Low Input”