C1A27

Code

Generic C — Chassis

ABS wheel speed sensor circuit intermittent (rear right)

Brand: Generic

Type: C — Chassis

Views: UK: 13 EN: 13 RU: 14

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Damaged or corroded sensor connector
Broken, chafed, or intermittent wiring in the harness (open/short/short-to-ground)
Faulty wheel speed sensor (Hall or variable reluctance)
Contaminated, damaged or missing tone/ring tooth on reluctor wheel
Poor sensor air gap or misalignment
Intermittent ABS module input, connector, or ground

Symptoms

ABS warning lamp (and possibly traction/stability lamp) illuminated intermittently
ABS or traction control may behave incorrectly or not engage on that wheel
Inconsistent or pulsing brake feel under ABS operation
Traction/stability interventions may be erratic or disabled
No obvious drivability symptoms at steady speeds until ABS activates

What to check

Read freeze-frame and live data with a scan tool; monitor rear-right wheel speed while spinning wheel or road test
Visual inspection of sensor, connector, harness routing, and reluctor/tone ring for damage, debris, rust or missing teeth
Wiggle the harness and connector while watching live data or checking for intermittent fault appearance
Check connector for corrosion, bent pins, water ingress, and secure fit
Inspect wheel bearing/play and rotor/reluctor mounting for excessive movement
Measure supply/reference and ground at the sensor connector (with connector connected/backprobing)

Signal parameters

Hall-effect (3‑wire) type: 5V reference present at connector, signal toggles between ~0.4–4.5 V (square wave) with wheel rotation; frequency increases with speed
Variable reluctance (2‑wire) type: AC waveform present when wheel spins; amplitude increases with speed (millivolts at low speed to volts at higher speed)
Typical sensor resistance: a finite resistance (usually hundreds to a few thousand ohms) — consult vehicle service data for exact spec
Expected behavior: clean, repeatable waveform without intermittent dropouts, noise spikes or DC offset when wheel rotates

Diagnostic algorithm

Connect a scan tool and confirm C1A27 is active or historic. Record freeze-frame and live rear-right wheel speed data. Clear codes and attempt to re-create.
Visually inspect rear-right sensor, reluctor/tone ring, wheel bearing, and wiring routing for damage, debris, or corrosion.
With connector connected, backprobe sensor power/reference and ground. Confirm stable reference voltage (~5V or vehicle-specific) and ground continuity to chassis.
Monitor sensor signal with an oscilloscope while spinning the wheel (or on-road while watching live data). Look for missing pulses, intermittent dropouts, excessive noise, or low amplitude.
Wiggle harness and connector while observing live data or oscilloscope for intermittent changes. Repair/secure any chafe points and retest.
If no output or abnormal output, disconnect sensor and measure sensor resistance (and polarity for VR) and compare to service spec. Replace sensor if out of range or open.
Inspect and repair/replace damaged connector pins, clean corrosion, and ensure proper sealing. Reassemble and retest.
If wiring or connector continuity fails, perform pin-to-module continuity test and repair broken wires or corrosion. After repairs clear codes and road test to confirm fault does not return.
If sensor and wiring verify good, consider ABS control module input fault — verify module grounds/power and consult module diagnostics before replacement.

Likely causes

Intermittent connector contact at rear-right sensor
Worn/damaged sensor or reluctor ring (metal debris, missing tooth)
Chafed wiring harness where it flexes near suspension
Ground or reference voltage intermittent at ABS module
Sensor contaminated with debris or metal filings

Fault status

⚠️ Status

C1A27 — ABS Wheel Speed Sensor Circuit Intermittent (Rear Right). Stored when the ECU detects intermittent or erratic wheel speed input from the rear-right sensor. May be logged as intermittent; code may clear if fault stops occurring.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours