What does it 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.
Symptoms
- ABS, traction and stability control lamps on, usually all three at once
- ABS switches off, so there is no pedal pulsing and the wheels can lock under hard braking
- ABS or traction control cutting in for no reason at very low speed just before the car stops
- Speedometer, cruise control or hill-start assist misbehaving on vehicles that use wheel speed data for them
- Live data shows the left front wheel reading zero or jumping wildly while the other three track road speed
Likely causes
- Wheel speed sensor tip covered in metallic debris, rust scale or brake dust, weakening the signal
- Sensor harness chafed or stretched where it flexes with the steering and suspension between the knuckle and the inner wing
- Water and corrosion inside the sensor connector behind the wheel arch liner, lifting the contact resistance
- Cracked, rusted or chipped reluctor ring, or one that has spun loose on the hub or driveshaft
- Sensor pushed out of position, or corrosion behind it in the hub bore opening up the air gap
- Worn wheel bearing letting the hub move enough to lose the signal under cornering load
- Internal failure of the sensor itself, or a fault in the ABS module input circuit
How to fix it
- Read live wheel speed data for all four wheels and drive slowly; the faulty corner is the one that reads zero, lags or spikes
- Inspect the sensor and its harness at the left front corner, paying attention to where the loom passes the strut and the steering knuckle
- Pull the sensor, clean the tip and its bore, and inspect the reluctor ring all the way round by turning the wheel by hand
- Unplug the connector and check for water and corrosion, then measure sensor resistance for a passive sensor or supply voltage and signal for an active one
- Check wheel bearing play and make sure the sensor sits fully home with the correct air gap
- Do a wiggle test on the harness with live data running to catch an intermittent break inside the insulation
- Repair or replace the harness section, then replace the sensor and, if the tone ring is damaged, the hub or bearing assembly