Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
The car's computer is saying that there isn't enough power getting to the sensor that tells how fast the right rear wheel is turning.
The C121B code indicates a problem with the voltage supply to the right rear wheel speed sensor, which is crucial for systems like ABS to function correctly.
The right rear wheel speed sensor is responsible for sending data about wheel speed to the vehicle's ABS system. If the supply voltage to this sensor is too low, it can cause incorrect readings or a failure to send data, which can affect braking performance.
• ABS warning light on
• Traction control light on
• Inoperative ABS system
• Faulty wheel speed sensor
• Damaged wiring or connectors
• Blown fuse related to the ABS system
ABS: Anti-lock Braking System
OBD-II: On-Board Diagnostics II
• Check the ABS fuse and replace if necessary.
• Inspect the wiring and connectors for damage or corrosion.
• Test the wheel speed sensor for proper operation.
• Replace the faulty wheel speed sensor.
• Repair or replace damaged wiring or connectors.
• Ensure all connections are clean and secure.
• Multimeter
• OBD-II scanner
• Basic hand tools
$100 - $300
Moderate
• Dodge Charger
• Jeep Grand Cherokee
• Chrysler 300
• Regularly inspect ABS wiring and connectors.
• Ensure the vehicle's electrical system is in good condition.
• Avoid driving through deep water which can damage sensors.
Sensor Issue
• Do not ignore the ABS warning light.
• Get the vehicle inspected by a professional if unsure.
• Drive cautiously until the issue is resolved.
Check with your vehicle manufacturer as ABS components may be covered under warranty.
This code has been commonly reported in vehicles with high mileage or those exposed to harsh weather conditions.
Moderate – It affects the ABS system which is important for safe braking.
Driving with a malfunctioning ABS system can increase stopping distances and reduce vehicle control during emergency braking.
• Vehicle stationary and ignition on.
• Check voltage supply with a multimeter.
The right rear wheel speed sensor is located near the wheel hub of the right rear wheel.
Q: Can I drive with a C121B code?
A: It's not recommended as it affects the ABS system, which is crucial for safe braking.
Q: How can I reset the ABS light?
A: Fix the underlying issue first, then use an OBD-II scanner to clear the code.
Pro · Four-system diagnostics
A handheld scanner aimed at engine, transmission, ABS, and airbag diagnostic work.
Chassis and ABS faults usually need a scanner that can read brake-system data directly.
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Universal · Torque wrench
Useful when reassembly torque matters for wheel, suspension, and under-hood fasteners.
Useful when wheel, hub, or suspension hardware needs correct reassembly torque.
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Universal · Drive-on ramps
Helpful for safer access underneath the vehicle during exhaust, EVAP, and underbody inspection work.
Helpful for safer access during underbody inspection work.
View on AmazonThese are optional diagnostic tools, not guaranteed fixes. As an Amazon Associate I earn from qualifying purchases.