C1237
Speed Wheel Rear 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

Your vehicle's ABS system can't read the speed signal from a rear wheel sensor, like a speedometer that suddenly goes blind to one wheel. Without this data, the ABS can't function properly and may trigger warning lights.

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Symptoms You May Notice
3 known symptoms for this code
ABS warning light illuminated on dashboard
Loss of ABS functionality during braking
Traction control disabled or inoperative
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ABS module monitors wheel speed signals from all four wheels to detect wheel lock during braking. When a rear wheel sensor signal drops below expected frequency or voltage thresholds, the ECU logs this fault. The system expects a consistent AC signal varying with wheel rotation speed.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Rear wheel signal frequency Signal present and proportional to wheel speed No signal or signal below 2 Hz at vehicle speed
Signal voltage 0.3-5V AC varying with speed Constant low voltage or no voltage detected
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Rear wheel speed sensor
Unplug the sensor connector and inspect for corrosion, then clean contacts with electrical contact cleaner and reconnect.
2
ABS wiring harness
Trace the rear wheel sensor wire from wheel hub to ABS module, checking for chafed insulation, pinched wires, or loose connectors.
3
Rear wheel speed sensor replacement
Remove the wheel, unbolt the sensor from the hub assembly, install a new sensor, and clear the fault code.