|Trouble Code||Fault Location||Probable Cause|
|C1155||Speed Wheel Sensor LF Input Circuit Failure|
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What Does Code C1155 Mean?
OBD II fault code C1155 is a manufacturer specific code that is defined by car makers Ford, Jaguar, Lincoln, Mercury, and Mazda as “Speed Wheel Sensor LF Input Circuit Failure”, and is set on these applications when the PCM (Powertrain Control Module) detects a failure, defect, or malfunction in the circuits associated with the left front wheel speed sensor.
Wheel speed sensors are typically of the Hall-effect type that generates magnetic fields when the teeth on a reluctor ring pass in front of the sensing element as the reluctor ring rotates with the wheel. Reluctor rings are typically attached to the driveshaft, but on some applications, the reluctor ring is incorporated into the wheel bearing.
Since the teeth on all reluctor rings that are associated with wheel speed sensors are evenly spaced around the circumference of the ring, the signals that are generated as the reluctor ring rotates are also evenly spaced. This makes it possible for a PCM to “count” the number of signals that are generated during each full revolution of a wheel, which number serves as the basis from which the PCM calculates the rotational speed of each individual wheel.
In practice, all wheel speed sensors are connected to the ABS brake control module first, before the connection is continued to the PCM. This is an important point, since the ABS brake control module uses the rotational speeds of each individual wheel to calculate appropriate braking forces to prevent one or more wheels from locking up on loose, wet, or slippery road surfaces.
As a practical matter, when the brakes are applied and it happens that one or more wheels lock up under severe braking, the ABS system compares the rotational speeds of all the wheels. In a fully functional system, the ABS control module will use the frequency of the signals generated by the wheel speed sensors to release the brakes on the wheels with the lowest rate of rotation several times per second. The effect of this is that in this manner, the ABS system is able to prevent one or more wheels from locking up, which ensures even braking while enabling a driver to steer the vehicle around obstacles safely.
Additionally, signals from the wheel speed sensor also enable the ABS system to supply selective braking forces to individual wheels when other safety systems, such as traction and stability control are in operation. In the case of traction control, the ABS supplies selective braking forces to spinning wheels to divert torque to non-spinning wheels, while in the case of stability control, the ABS system supplies selective braking forces to individual wheels to force the vehicle to follow the line dictated by the steering angle, and other sensors such as yaw rate sensors and lateral accelerometers.
All of the critical safety systems depend on accurate signals being generated by the wheel speed sensors to function as designed. Therefore, if a failure, defect, or malfunction exists either in a wheel speed sensor itself, or in a associated electric circuit, the PCM and/or ABS control module will recognize that neither the ABS system, nor several other safety systems can be controlled effectively and either one or both control modules will set code C1155, and illuminate one or more warning lights as a result.
Where is the C1155 sensor located?
The image above shows the location (circled) of the wheel speed sensor on a front wheel of a Ford Escape model. Note that the actual appearance and location of wheel speed sensors vary somewhat between applications, but in all cases, the sensor will be located close to a driveshaft. The easiest way to locate wheel speed sensors is to look for electrical connectors on wheel hubs, such as shown in the above example.
What are the common causes of code C1155?
The most common causes of code C1155 are much the same across all affected applications, and could include one or more of the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the affected wheel speed sensors’ circuitry
- Damaged or defective wheel speed sensor
- Damaged or defective ABS brake control module
- Failed or failing PCM. but note that since this is a rare event, the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code C1155?
Common symptoms of code C1155 could include one or more of the following-
- Stored trouble code and illuminated warning light
- In some cases, multiple additional codes, and several warning lights, including traction control and stability control warning lights may be illuminated
- ABS braking may not be available, but note that normal braking is usually not affected
- Traction control, stability control, and cruise control systems may not be available