AutoCodes – Generic – C120B

C120B Code - Ignition Supply Voltage Low

Quick Answer

The Diagnostic Trouble Code (DTC) will be set when the Master Cylinder Pressure Sensor power supply voltage decrease occurs, or a history of voltage decrease exists, and defective Master Cylinder Pressure Sensor output continues for 1 second or more. Common causes include Faulty Master Cylinder Pressure Sensor, Master Cylinder Pressure Sensor harness is open or shorted, Master Cylinder Pressure Sensor circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: C120B indicates Ignition Supply Voltage Low.
Common causes: Common causes include Faulty Master Cylinder Pressure Sensor, Master Cylinder Pressure Sensor harness is open or shorted, Master Cylinder Pressure Sensor circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

Save to My Repairs My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 381

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jan 18, 2023

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

Code C120B Description

OBDII Code C120B - Ignition Supply Voltage Low - AutoCodes.com — C120B Code - Ignition Supply Voltage Low

What does this mean?

What are the Possible Causes of the DTC C120B?

Faulty Master Cylinder Pressure Sensor
Master Cylinder Pressure Sensor harness is open or shorted
Master Cylinder Pressure Sensor circuit poor electrical connection
Faulty Skid Control ECU (Brake Actuator Assembly)

How to Fix the DTC C120B?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the C120B code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the C120B code?

Need more information on how to fix the C120B code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the C120B code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

C120B Lexus — Ignition Supply Voltage Low
C120B Scion — Ignition Supply Voltage Low
C120B Toyota — Ignition Supply Voltage Low

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about C120B Code

1. What does the OBDII code C120B mean?

C120B indicates that the ignition supply voltage is below the required level for proper operation of the vehicle's systems, which can affect various components, especially those in the ABS or stability control system.

2. What are the common symptoms of the C120B code?

Symptoms may include the ABS or traction control warning light on, erratic ABS or stability control behavior, and difficulty starting the vehicle or intermittent electrical issues.

3. What causes the C120B code to appear?

Common causes include a weak or failing battery, loose or corroded battery terminals, a faulty alternator, damaged wiring, or issues with the ignition switch.

4. How serious is the C120B code?

While the vehicle may still be drivable, this code can affect critical safety systems like ABS and traction control. It should be addressed promptly to avoid further electrical issues or compromised safety.

5. Can a low battery trigger the C120B code?

Yes, a low or failing battery is a common cause of the C120B code because insufficient voltage can disrupt the operation of the ignition and other systems.

6. How can I fix the C120B code?

Start by checking the battery’s charge and condition, inspect and clean the battery terminals, verify the alternator’s output, and examine the wiring and connections for damage or corrosion.

7. Can I clear the C120B code without fixing the problem?

You can clear the code with an OBDII scanner, but if the underlying issue isn’t resolved, the code will likely return and could lead to further electrical or safety system problems.

8. How do I test the alternator if I have a C120B code?

Use a multimeter to check the alternator’s output voltage. It should typically read between 13.5 and 14.7 volts when the engine is running. Anything outside this range may indicate a faulty alternator.

9. Is a damaged ignition switch a potential cause of the C120B code?

Yes, a damaged or failing ignition switch can cause low voltage to critical systems, potentially triggering the C120B code.

10. Can driving with the C120B code damage my vehicle?

While it may not immediately harm the vehicle, prolonged driving with this issue can lead to further electrical problems or disable key safety systems like ABS, increasing the risk of an accident.