|Trouble Code||Fault Location||Probable Cause|
|U0124||Data bus: lateral acceleration sensor module - no communication|
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What Does Code U0124 Mean?
OBD II fault code U0124 is defined as “Data bus: Lateral Acceleration Sensor Module – no communication”, and is set when the PCM (Powertrain Control Module) does not receive any communication from the lateral acceleration sensor via the ABS/ESC control module for a predefined period of time. Note that depending on the application, the time needed for communication to be lost before the code sets varies. Typically though, an interruption in communication of once in about 5 seconds will set the code on most applications.
On vehicles that are fitted with electronic stability control, the function of the lateral acceleration sensor is to detect accelerations that deviate from the line set by the steering angle. When the steering wheel is turned, the steering angle sensor communicates the steering angle to various other sensors, including the lateral acceleration sensor and the ABS controller.
If the vehicle is operated normally, the vehicle will follow the line dictated by the front wheels; however, should the cornering forces exceed the abilities of the vehicle to cope with those forces, the accelerations detected by the lateral acceleration sensor will trigger the ABS system to apply braking forces to the wheels selectively to force the vehicle to follow the line dictated by the steering angle.
In practical terms, the lateral acceleration sensor measures sideways movements of the vehicle, and should a potentially dangerous situation arise, the ABS brake system is activated autonomously to prevent a vehicle from entering a slide or skid- sometimes even before the driver realizes that a dangerous situation is developing. In practice, the ABS system is used to slow the rotation of selected wheels, thus serving as “anchors” around which to pivot the vehicle to maintain stability during evasive manoeuvres or emergency braking.
Note however that the lateral acceleration sensor is programmed to assume that the vehicle is in good mechanical condition. It cannot detect bald tires, worn suspension components, or compensate for reckless driving. Therefore, since no safety system can overcome the laws of physics, an over reliance on the stability control system should be avoided.
The image below shows the typical configuration of a stability control system, with reference to all the other main sensors and controllers that are involved in making the system work. Note the location of the lateral acceleration sensor in this application. This is only one possible location for this sensor; locations differ widely, and it could be anywhere from under the dashboard, to under any of the seats. Consult the manual for your vehicle to locate the various sensors.
What are the common causes of code U0124?
The most common cause of this code is shorted, burnt, damaged, or corroded wiring/connectors, or other damage to wiring that causes open circuits. Other possible causes could include the following-
Defective lateral acceleration sensor
Defective lateral acceleration sensor control module. This is a relatively rare event, and it is far more likely that the sensor will fail before the control module does.
Defective ABS wheel speed sensors. Note that if wheel speed sensors are implicated in this code, it is almost certain that wheel speed sensor related codes will be present along with U0124.
Voltage spikes caused by incorrect jump starting procedures have the potential to damage almost any controller in any vehicle, including sensors like the lateral acceleration sensor and its control circuit. Check for other codes that might have been caused by a voltage spike.
Low battery voltages due to defective batteries can have unpredictable results, including damaging sensors and control circuits. Check for the presence of other codes that might have been set by low battery voltages.
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 U0124?
It is rare for symptoms other than a stored code and an illuminated warning light to be present. In the vast majority of cases, the affected system, in this case the electronic stability control system will be deactivated.
How do you troubleshoot code U0124?
WARNING: Diagnosing problems in any vehicle’s CAN (Controller Area Network) systems is a highly technical process, and specialized knowledge of automotive electronics and diagnostic procedures is a requirement to prevent secondary damage caused by ill-considered or wrong diagnostic methods. Diagnosis and repair of CAN systems should ideally not be attempted by non-professional persons.
Be aware that the diagnostic and repair procedure outlined below should not be seen as anything else than a general outline. Diagnostic and repair procedures for code U0124 are often make and model specific, and specialized equipment may be required to accurately diagnose this code on some applications.
NOTE 1: If you are confident enough in your diagnostic abilities to attempt the diagnosis (despite the above warning) you will need a repair manual, a good quality digital multimeter, and a color-coded wiring diagram since on some applications, the sensor harness/connectors sometimes contains two or more wires with the same thickness and color. Divining the purpose of similar wires is impossible without the benefit of a wiring diagram that clearly shows the function, routing, connections, and final termination(s) of these wires.
Record all fault codes present, as well as all available freeze frame data. This information could be useful should an intermittent fault be diagnosed later on.
NOTE: If any other codes relating to the ABS or other CAN system(s) are present, diagnose and resolve these codes first, before attempting to diagnose code U0124. Also note that on some applications, problems with wheel speed sensors or the ABS controller can set code U0124.
If no other codes are present, consult the manual to locate the lateral accelerator sensor. On most applications the sensor is located inside the passenger cabin, but there are still a few applications where the sensor is located outside of the cabin, such as in the trunk.
Look for visible damage to the sensor. Note that damage may not always be visible; if the sensor is not damaged by an impact or flooding, consult the manual on the location, color-coding, function, and routing of all associated wiring. Look for shorted, burnt, broken, corroded, or otherwise damaged wiring and/or connectors.
NOTE: If damaged wiring is found, resist the temptation to repair the wiring. Since some CAN systems can be very sensitive to variations in resistance and continuity, the wiser option is always to replace the relevant harness with an OEM replacement to avoid issues and problems later on.
If no visible wiring issues are found, consult the manual on the procedures for testing resistance, reference voltage, and ground values in all associated wiring. On most applications, the reference voltage for the lateral acceleration sensor is 5 volts, but this may vary between applications. Note that a reference voltage over or under specified values can also sometimes set this code. Consult the manual on the exact values that apply to the application being worked on.
Compare obtained readings to those stated in the manual, and replace the harness if any deviations are found. Retest the system after replacing the harness to ensure that all readings fall within the manufacturers specifications.
If there is any doubt that the sensor is working correctly, replace it. Do NOT attempt to open or “repair” this sensor if the manual does not clearly describe testing procedures. If the manual does not describe such a procedure, it is safe to assume that the sensor cannot be tested reliably.
If all electrical values fall within specifications, it is likely that the sensor itself is faulty. However, due to the design of lateral acceleration sensors, it is for the most part impossible to test these sensors apart from checking their internal resistance. Signal voltages are dependent on the design and programming of the sensor controller, meaning that shaking it about to see if it generates a signal voltage is NOT a reliable way to test this sensor.
NOTE: The signal voltage generated is dependent on the direction and amount of lateral acceleration, as well as the vehicle’s road speed. On most applications, this sensor remains inactive until the road speed exceeds about 25 km/hour, but this speed also varies between applications. For these reasons, it is very difficult to measure signal voltages accurately, since most software used in generic code readers cannot monitor live data from this sensor reliably, or accurately. Therefore, shaking the sensor about to test for signal voltages is NOT recommended, since the normal operating parameters/conditions of the sensor cannot be replicated accurately.
After replacing the sensor and/or the wiring harness, clear the code, and operate the vehicle to see if the code returns. However, resist the temptation to drive aggressively to see if the stability control system works. If the replacement lateral acceleration sensor works as intended, it will communicate that fact to the PCM via its controller. If on the other hand, it does not work, that fact will also be communicated to the PCM.
If the code does not return after a few cycles of normal operation, it is safe to assume that the problem has been resolved.
If the code does return after a few normal drive cycles, there may be an intermittent fault present. However, unless you possess above average diagnostic skills, resist the temptation to try and diagnose intermittent faults in any CAN or Data Bus system.
Given the complexity of this type of control system, it is strongly recommended that the vehicle be referred to a specialist for professional diagnosis and repair.
Codes Related to U0124
While code U0124 usually indicates a no-communication problem with the lateral acceleration sensor, this is not the case on all applications. On some vehicles, code U0124 is an engine code that indicates a fuel/air metering problem. It is therefore recommended that the exact definition of code U0124 as it applies to the application being worked on, be researched before commencing a diagnostic procedure.