C0042 — Left-Front (LF) Wheel Speed Sensor: Range/Performance

What it means

C0042 sets when the ABS/EBCM sees the LF wheel-speed signal out of the expected range for actual vehicle conditions (implausible, intermittent, too low amplitude, noisy) as compared with the other three sensors and vehicle speed. On many GM platforms, the ABS module supplies ignition voltage to active wheel-speed sensors and expects a DC square-wave return that scales with wheel speed; out-of-range behavior triggers this DTC. Charm

Typical symptoms

  • ABS lamp on; StabiliTrak/ESC/Traction features limited or disabled.
  • Possible low-speed ABS pulsation due to a noisy/weak signal.

Why it sets (typical OEM criteria)

  • The EBCM detects an implausible LF speed vs. the other wheels for a calibrated time/distance, or
  • The LF signal amplitude/frequency is degraded or intermittent relative to the other wheels.
    (Exact timers/thresholds are OEM-specific, but GM SI describes active WSS behavior and EBCM expectations for square-wave integrity.) Charm

Common root causes

  • Sensor output too weak/erratic: debris on the sensor tip; excessive air gap; mis-seated sensor.
  • Encoder (tone ring) faults in the hub bearing (cracked/missing segments, “rust jacking”).
  • Harness/connector issues at the knuckle/intermediate junction (chafe, corrosion, water intrusion, poor pin tension).
  • Less common: EBCM input circuit fault. CarParts+1

Professional diagnostics (step-by-step)

  1. Scan-tool triage
    • Pull DTCs; go to live data and graph all four wheel speeds. Drive 10–30 mph steady.
    • The LF that drops out, reads implausibly low, or chatters while others are stable indicates a performance fault at that corner. (Compare to RF/LR/RR traces).
  2. Visual & mechanical checks (wheel off)
    • Verify sensor seating and mounting surface; clean the tip.
    • Inspect the hub encoder (if integrated): look for cracking, corrosion ridges, or missing magnetic segments. Replace the hub if compromised.
  3. Electrical tests
    • For active (Hall) sensors: key-on, verify supply and ground at the connector; back-probe the signal and confirm a clean square-wave that scales with speed.
    • For passive (inductive) sensors (older platforms): spin the wheel and check AC amplitude—GM TSB guidance expects >~100 mV AC at modest speed; a weak waveform points to gap/encoder issues or a marginal sensor. TSB Search
  4. Harness integrity
    • Unplug the knuckle connector and the next inline connector; check for high resistance, opens, shorts-to-ground/power.
    • Pay special attention to the short pigtail at the knuckle and the fender-liner junction, known failure points on GM/Ford trucks/SUVs. CarParts
  5. Module side
    • If the sensor and harness produce a clean signal but the fault persists, follow SI to evaluate the EBCM channel.

Verified fixes

  • Clean/reseat or replace the LF wheel-speed sensor; set the correct air gap.
  • Replace the hub/bearing if the encoder is damaged.
  • Repair/repin corroded or loose connectors; repair chafed sections; restore sealing/strain relief.
  • Replace the EBCM/Modulator only if all inputs are good and diagnostics point to an internal channel fault. Charm+1

Sources:

  • GM Service Information excerpt (Charm.li) describing active WSS behavior (EBCM supplies voltage; sensor returns square-wave) and diagnostic expectations for wheel-speed inputs. Charm
  • GM/Chevrolet TSB guidance on minimum WSS AC amplitude (~100 mV) as a diagnostic threshold (used on inductive implementations). TSB Search
  • Technical overviews of common wheel-speed sensor and encoder failure modes/symptoms used by ABS/ESC. CarParts