C0502

Code

Generic C — Chassis

Left Front Wheel Speed Sensor A Circuit Low

Brand: Generic

Type: C — Chassis

Views: UK: 27 EN: 66 RU: 32

AI status

Completed

ready

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Page language: EN

Causes

Damaged or corroded sensor connector or wiring (short to ground)
Failed wheel speed sensor (passive or active)
Broken or damaged tone ring (missing teeth, heavy rust/debris)
ABS module or harness short/internal fault
Poor ground or supply voltage to active sensors
Intermittent faults caused by wheel bearing play or physical damage

Symptoms

ABS warning light illuminated
Traction/stability control warning or disabled
Speedometer irregular or intermittent (on some vehicles)
Reduced ABS/ESC performance or unexpected ABS activation
Possible diagnostic trouble codes for other wheel sensors or ABS module comms

What to check

Scan with a capable scan tool: read C0502, freeze frame and live wheel speed values
Visually inspect left front sensor, connector, wiring loom and tone ring for damage or contamination
Backprobe sensor connector: check supply (if active), ground and signal while spinning wheel
Measure sensor resistance (passive type) and compare to manufacturer spec
Measure AC voltage output of passive sensor while turning wheel (~0.1–2 VAC or higher depending on speed)
Check for continuity to ABS module connector and possible short to ground/power

Signal parameters

Passive (magnetic) sensor: DC resistance typically ~500–2000 Ω depending on design; AC output increases with speed, commonly ~0.1–2.0 VAC at low crawl speeds and higher at road speeds; waveform sinusoidal/AC.
Active (Hall/IC) sensor: requires supply (commonly 5V or 12V switched/regulator); output is a pulsed square wave from ~0 V to supply voltage (0–5V) with frequency proportional to wheel speed.
Expected behavior: clean waveform with amplitude and frequency increasing smoothly with wheel rotation; sudden drop toward 0 V, very low AC amplitude, or constant voltage indicates a fault.
If measured signal is below ~0.1 VAC for passive sensors at moderate wheel speed or output stuck near ground for active sensors, suspect a low/open/shorted circuit or failed sensor.

Diagnostic algorithm

Confirm DTC: clear codes, test-drive or spin the wheel to re-create C0502. Note freeze frame/live data.
Visual inspection: examine left front sensor, connector, tone ring, and wiring for damage, corrosion, contamination or rubbed-through insulation.
Perform connector checks: disconnect sensor and inspect pins; check for bent pins, corrosion, water ingress, or pushed-out terminals.
Measure sensor resistance (passive): with connector disconnected and multimeter on Ohms, compare reading to spec; infinite or very high = open, very low = short.
Measure signal while spinning wheel: for passive use AC voltmeter or scope; for active backprobe for supply and measure output with oscilloscope or DVM (frequency/voltage).
Check power/ground (active sensors): verify supply voltage present with ignition on and good ground reference at connector.
Check continuity from sensor connector to ABS module: verify no short to chassis ground or power, and acceptable resistance/backprobing to module pins.
Inspect tone ring: check for missing teeth, heavy rust, or debris; if tone ring damaged, replace and clean mounting area.
If wiring and sensor check OK, test ABS module inputs and grounds; substitute known-good sensor (if available) to confirm behavior.
Repair: replace faulty sensor, repair wiring/connector, clean/replace tone ring or repair ABS module wiring as determined. Clear codes and retest to verify repair.

Likely causes

Connector corrosion or pin pushed out at the wheel hub
Open or short in the sensor harness near the wheel (bent, chafed)
Failed wheel speed sensor (most common)
Contaminated or damaged tone ring reducing AC amplitude
Loose/poor ground or blown fuse feeding the sensor (active type)

Fault status

⚠️ Status

Left Front Wheel Speed Sensor A Circuit Low — low voltage or short to ground detected on left front wheel speed sensor circuit. ABS/ESC functionality may be limited.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours