Speed Wheel LF Input Signal Missing

OBD II fault code C1233 is a manufacturer-specific trouble code that is defined by carmakers Ford, Lincoln, Mercury, and Mazda as “Left Front Wheel Speed Sensor Input Signal Missing”, and is set on these applications when the PCM (Powertrain Control Module) detects the lack of an input signal from the left front wheel speed sensor. NOTE: Carmaker Jaguar has assigned a similar definition to this code, but with this difference; “Front left sensor – long term detection of missing sensor signal”.

Despite the slight difference in wording, the causes, symptoms, and diagnostics for this code are largely identical on affected applications. Also known as, “ABS sensors” the purpose of wheel speed sensors is to measure the rotational speed of the wheels on a vehicle that is in motion, and they do this for several reasons.

Arguably, the most important reason is that the ABS system uses this information to modulate braking forces to individual wheels to prevent one or more wheels from locking up under aggressive braking. In practice, the sensor generates magnetic signals as the teeth on a reluctor ring pass in front of the sensor. The PCM converts the magnetic signals into electrical signals, and in a fully functional system, each tooth on a reluctor ring will generate a signal.

Thus, as a practical matter, the PCM both counts the number of signals coming from each wheel and monitors the frequency of the signals, with the frequency of the signals correlating directly to the rotational speed of each wheel. Assuming then that both the wheel speed sensors and the reluctor wheels on all road wheels are in perfect working condition, it is a simple matter for the PCM to compare the rotational speeds of the wheels directly, and in real-time.

Therefore, should one or more wheels begin to lock-up under aggressive braking, the PCM will “see” this in the different rates at which the affected wheels rotate, as compared to the wheels that are not threatening to lock-up. To prevent the wheels from actually locking up, the ABS control module will release and reapply appropriate braking forces to the affected wheels up to about 12 times per second and it will continue to do this until all the wheels again rotate at the same rate.

The biggest principal advantages of brake modulation are a) that tires are prevented from losing traction and, b) that by maintaining traction, a driver can safely steer the vehicle around obstacles. While Traction-, and Stability Control systems also use the ABS system to work, these systems apply differential braking forces to individual wheels to maintain and/or restore a vehicle’s stability under some driving conditions.

In practice, these systems use the same components as the ABS system, and therefore, these safety systems (and several others) also depend on receiving accurate information about the rate at which individual wheels rotate.

Under some conditions, some aspects of the ABS, Traction-, and Stability Control systems can all be in operation at the same time, such as during, for instance, a violent evasive maneuver that displaces the vehicles’ weight from side to side, or from front to rear (or both at the same time).

Maintaining both a vehicle’s stability and exerting effective directional control at the same time during these kinds of maneuvers are typically beyond the capabilities of most average drivers, which illustrates the importance of electronic interventions that depend on accurate input data from wheel speed sensors to be effective.

Thus, in practice, if a PCM detects the lack of an input signal from a wheel speed sensor, which in this case, is the wheel speed sensor on the left front wheel, it will recognize that it cannot control, manage, or monitor the operation of the ABS and other safety systems effectively, and it will set code C1233 and illuminate one or more warning lights as a result.