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 the power going to the sensor that tells how fast the back left wheel is spinning is too low.
The vehicle's control module has detected that the voltage supply to the left rear wheel speed sensor is below the expected level.
The C121A code indicates an issue with the electrical circuit that supplies voltage to the left rear wheel speed sensor. This sensor is crucial for the anti-lock braking system (ABS) and traction control system, as it monitors the rotational speed of the wheel. A low voltage condition can lead to inaccurate wheel speed readings.
• ABS warning light on the dashboard
• Traction control light illuminated
• Inconsistent braking performance
• Faulty left rear wheel speed sensor
• Damaged wiring or connectors
• Faulty ABS control module
ABS: Anti-lock Braking System
OBD-II: On-Board Diagnostics II
• Inspect the left rear wheel speed sensor and its wiring for damage.
• Check the voltage supply to the sensor using a multimeter.
• Test the ABS control module for faults.
• Replace the left rear wheel speed sensor if faulty.
• Repair or replace damaged wiring or connectors.
• Replace the ABS control module if necessary.
• Multimeter
• OBD-II scanner
• Basic hand tools
$100 - $300
Moderate
• Dodge Charger
• Jeep Grand Cherokee
• Chrysler 300
• Regularly inspect wheel speed sensors and wiring.
• Avoid driving through deep water which can damage electrical components.
• Schedule routine maintenance checks.
Sensor Issue
• Drive cautiously if the ABS light is on.
• Have the vehicle inspected by a professional as soon as possible.
Check with the vehicle manufacturer for warranty coverage on ABS components.
This code has been associated with issues in vehicles with older ABS systems where wiring degradation is common.
Moderate – While the vehicle is still drivable, the ABS and traction control systems may not function properly.
Driving with a malfunctioning ABS can increase the risk of losing control during braking.
• Vehicle at rest
• Ignition in the 'on' position
The left rear wheel speed sensor is located near the hub of the left rear wheel.
Q: Can I drive with the C121A code?
A: Yes, but it's not recommended as the ABS and traction control systems may not work properly.
Q: How do 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.
View on Amazon
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.
View on Amazon
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.