Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This code means that the part of the car that helps you press the brakes easily isn't working well. It's like when you use a bicycle pump and it doesn't push air smoothly.
The brake booster helps reduce the effort needed to apply the brakes. If it's not performing well, it can affect braking efficiency.
The C0021 code is triggered when the vehicle's control module detects a problem with the brake booster system, which is responsible for amplifying the force applied to the brake pedal. This could be due to a vacuum leak, sensor issue, or mechanical failure.
• Hard brake pedal
• Increased stopping distance
• Brake warning light on
• Vacuum leak in the brake booster
• Faulty brake booster sensor
• Mechanical failure of the brake booster
Brake Booster: A device that uses vacuum to amplify the force applied to the brake pedal.
Vacuum Leak: A break or hole in the vacuum system that can cause performance issues.
• Inspect the brake booster for visible damage or leaks.
• Check for vacuum leaks in the system.
• Test the brake booster sensor for proper operation.
• Replace the brake booster if faulty.
• Repair or replace any damaged vacuum lines.
• Replace the brake booster sensor if defective.
• Vacuum gauge
• Multimeter
• Wrench set
$200 - $600
Moderate
• Ford F-150
• Toyota Camry
• Honda Accord
• Regularly inspect brake system components.
• Ensure proper maintenance of vacuum lines.
• Check brake booster operation during routine service.
Brake System Issue
• Have your brakes inspected regularly.
• Address any brake warning lights immediately.
Brake system components may be covered under the vehicle's powertrain warranty. Check with your manufacturer.
This code has been associated with vehicles experiencing hard braking issues due to vacuum leaks or sensor failures.
High – Brake performance is critical for vehicle safety.
Driving with impaired brakes can lead to accidents. Seek professional repair immediately.
• Engine running
• Brake pedal applied
The brake booster is typically located between the brake pedal and the master cylinder, near the firewall in the engine bay.
Q: What happens if I ignore this code?
A: Ignoring this code can lead to decreased braking performance and increased stopping distances, which could be dangerous.
Q: Can I drive with the C0021 code?
A: It's not recommended to drive with this code as it affects braking performance. Get it checked as soon as possible.
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.