C1261
Speed Wheel LR Comparison Failure
Chassis Chassis/Safety ABS Wheel Speed Comparison 🔴 Serious — Stop or limit driving
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What This Actually Means
In plain language — no jargon

The left rear wheel speed sensor isn't matching the other wheels' speeds, like one wheel is spinning differently than it should. The ABS system detected this mismatch and can't trust the sensor data to safely control braking.

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Symptoms You May Notice
3 known symptoms for this code
ABS warning light illuminated on dashboard
Vehicle pulls to one side during hard braking
ABS system disabled or grinding noise when braking
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU compares wheel speed signals from all four wheel speed sensors to detect skidding and control ABS braking. When the left rear wheel speed deviates significantly from the expected range based on front wheel speeds and vehicle dynamics, a comparison failure is triggered. The sensor signal must fall within a calculated threshold window.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
LR wheel speed variance vs. average Within 5-10% of front wheel speeds Exceeds 15% deviation or signal dropout
LR sensor signal amplitude 0.5-2.0V AC signal detected Signal below 0.3V or erratic/missing
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Left rear wheel speed sensor
Unplug the sensor connector at the wheel hub, clean corroded terminals with contact cleaner, and reinstall to eliminate poor connections.
2
Wheel bearing assembly (LR)
A worn bearing creates inconsistent wheel speed signals; inspect for play and replace if damaged or excessively worn.
3
Sensor wiring harness (LR brake line routing)
Check wiring for pinches or cuts near the left rear wheel that could interrupt signal transmission and repair or replace the damaged section.