C0042

C0042 is the erratic-signal cousin of C0041 — the EBCM is seeing a wheel-speed signal from the LR sensor, but the signal is inconsistent with the other three wheels (intermittent dropouts, signal noise, or speed reading that doesn't track vehicle speed). This usually points at a partial fault rather than a hard open: damaged tone ring teeth, excess air gap, contaminated sensor tip with metal debris, or a high-resistance connector. Cheapest-first ladder: (1) Live-data the LR WSS during a road test alongside the other three wheels — a healthy circuit tracks within 1-2 mph of the diagonal wheel during steady-state cruise; C0042 typically shows the LR dropping to zero or spiking high while the other three remain smooth. Passive sensors should still produce 0.4-1.5V AC at slow rotation building cleanly with speed; intermittent zero-crossings on the scope trace mean a damaged tone ring tooth or metallic debris bridging the air gap. Active sensors should show a clean square wave with consistent amplitude — missing pulses or jagged edges on the scope mean a marginal sensor or wiring problem. (2) Pull the sensor and inspect the magnetized tip for ferrous debris (a common find — brake-dust shavings stick to the magnet and degrade the signal); wipe it clean and clear the code before condemning the sensor. (3) Inspect the tone ring teeth for impact damage or missing/bent segments — one cracked tooth produces a dropout pulse every wheel revolution that the EBCM sees as erratic. (4) Wiggle-test the harness from sensor to module with the scan tool live — intermittent flat-lines under wiggle is a broken conductor inside the loom. Expensive-misdiagnosis caveat: GM trucks 1999-2007 again — C0042 here almost always traces to a hairline crack in the stamped-steel tone ring rather than the sensor; the ring still spins but loses one or two teeth to corrosion, and a brand-new $80 WSS does nothing to fix a tone-ring tooth that no longer exists.

Vehicle-specific patterns: 1999-2007 GM trucks with the Kelsey-Hayes 325 EBCM throw C0042 from partially-corroded tone rings (the ring is still attached but several teeth have rusted thin and produce weak pulses) — visual inspection with a flashlight through the backing plate confirms it before any parts come off. 1997-2014 Ford F-150 with integrated rear hub-mounted WSS throws C0042 when the wheel bearing develops play and the rotor-to-encoder air gap walks during cornering — diagnostic confirmation is grabbing the tire at 12 and 6 o'clock and feeling for bearing slop before quoting a hub. 2008-2014 Dodge Caravan throws C0042 from harness chafe at the trailing-arm clip (same location as C0041, just a partial wear-through instead of a full break). 2005-2012 Nissan Frontier/Pathfinder throws C0042 from a worn rear leaf-spring bushing that lets the axle walk side-to-side under braking, momentarily widening the WSS air gap past spec. Estimated repair: $80 to $450.