C1234: How to Diagnose & Fix — Right Front Wheel Speed Sensor Input Signal Missing

A number of K5 owners have reported premature rear wheel bearing failure, typically presenting as a low-pitched humming or growling noise that changes with vehicle speed and varies when turning left or right. The issue has been observed on both FWD and AWD variants and appears linked to bearing press-fit tolerances in the rear knuckle assembly. NHTSA complaints and owner forums document failures occurring well before expected service life, sometimes as early as 25,000 miles.

Sealed hub bearing assemblies on the Flex wear out and produce a speed-dependent noise that owners consistently describe as starting like a hum and progressing to a sound like aggressive snow tires, usually first noticed above 40 mph and changing pitch when the vehicle is loaded up in a turn. It affects both front and rear (including rear hubs on AWD cars). What makes this repair worse than on other vehicles is corrosion: the bearing race seizes to the knuckle/spindle, so the whole spindle often has to come off the car and the bearing pressed out, turning a routine job into a multi-hour fight.

Rear wheel bearing and hub assemblies are a frequent wear/failure item on independent rear suspension Explorers from this period. Owners commonly report a humming or growling noise that increases with road speed and can be mistaken for tire noise or differential problems. In some cases, excessive play also affects wheel speed sensor readings and can trigger ABS or traction control warnings.

Freestar owners report the ABS warning light coming on — typically after a corner — and staying on, which on this van also disables cruise control because the module drops out of the system. On vans with rear drum brakes, the rear wheel speed sensors sit where they collect metallic filings shed by the brake shoes; the accumulated ferrous debris bridges the gap to the tone ring and corrupts the speed signal. A related complaint is ABS activating at very low speed on dry pavement, which is the module reacting to a dropped-out signal from one wheel rather than to actual lock-up. A cracked or missing-tooth tone ring produces the same signature, as does a worn wheel bearing that lets the sensor gap open up.