How severe is fault code C1237?

Serious — Stop or limit driving

OBD Code C1237: Speed Wheel Rear Input Signal Missing

Q: What does OBD code C1237 mean?

Speed Wheel Rear Input Signal Missing. 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.

Q: What are the symptoms of fault code C1237?

ABS warning light illuminated on dashboard, Loss of ABS functionality during braking, Traction control disabled or inoperative

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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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How Your ECU Detects This

Technical sensor logic and voltage thresholds

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.

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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code C1237 is classified as a serious fault. If your check engine light is flashing — not just steady — pull over safely and do not continue driving. A flashing CEL indicates an active misfire or critical failure that can cause catalytic converter damage within minutes or permanent engine harm within miles. Contact a certified mechanic immediately. Do not attempt roadside repairs on high-severity codes unless you are trained to do so.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.

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How to Clear Code C1237

What happens after you fix the fault

Once the fault is repaired, C1237 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When

Fault has been diagnosed and repaired
You want to confirm the repair worked
Code appeared after a sensor was cleaned

⚠️ Do Not Clear When

Preparing for an emissions/PUC test
Root cause is still undiagnosed
Check engine light is flashing

Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.