C1236
Speed Wheel LR Input Signal Missing
Chassis Chassis/Safety ABS Wheel Speed Sensor 🔴 Serious — Stop or limit driving
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What This Actually Means
In plain language — no jargon

The ABS system can't read the speed signal from your left rear wheel sensor, like a speedometer that suddenly goes blind on one wheel. Without this data, the ABS can't properly control braking on that wheel.

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Symptoms You May Notice
3 known symptoms for this code
ABS warning light illuminated on dashboard
ABS system disabled or reduced braking performance
Traction control malfunction or loss of stability control
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ABS module monitors voltage signals from the left rear wheel speed sensor, which generates a frequency proportional to wheel rotation. When the sensor signal drops below detectable levels or becomes intermittent, the ECU logs this fault. The system expects a continuous AC signal within a specific frequency range during vehicle movement.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Wheel speed signal voltage 0.5-5V AC signal present No signal or below 0.1V for >2 seconds during motion
Signal frequency consistency Continuous pulse pattern matching other wheels Intermittent dropout or erratic pattern
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Left rear wheel speed sensor connector
Inspect and reseat the connector at the sensor, cleaning corrosion with electrical contact cleaner.
2
Left rear wheel speed sensor wiring harness
Check for damaged, pinched, or corroded wires between the sensor and ABS module, repairing as needed.
3
Left rear wheel speed sensor
Replace the sensor if connector and wiring are intact but signal remains absent.