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C0021 Brake Booster Performance (Subfault)
The C0021 Brake Booster Performance (Subfault) diagnostic trouble code indicates that the vehicle's Brake Control Module, ABS Control Module, or Electronic Brake Booster Control Module has detected that the brake booster is not providing the expected level of brake assist.
Unlike electrical circuit fault codes, C0021 is a performance-related diagnostic code. This means the brake booster is operating, but its overall performance does not meet the specifications programmed into the control module.
Modern vehicles continuously monitor brake booster operation using information from multiple sensors, including brake pressure sensors, vacuum sensors, brake pedal position sensors, and booster travel sensors. If the measured brake assist differs from the expected value, the control module stores the C0021 diagnostic trouble code and may illuminate one or more warning lights.
The word "Subfault" means the manufacturer has identified a more specific internal performance problem within the brake booster system. The exact meaning varies slightly between manufacturers but generally refers to insufficient or abnormal brake booster operation.
What Does C0021 Mean?
The brake booster reduces the amount of force required on the brake pedal.
The control module continuously evaluates:
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Brake assist level
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Vacuum or hydraulic pressure
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Brake pedal force
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Brake pedal travel
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Booster response
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Brake pressure generation
If the booster fails to provide the expected amount of assistance under normal operating conditions, the control module stores C0021.
Possible performance problems include:
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Weak brake assist
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Slow booster response
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Insufficient vacuum
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Internal booster leakage
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Hydraulic pressure loss
What Does the Brake Booster Do?
The brake booster multiplies the driver's braking force.
Depending on the vehicle, the system may use:
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Engine vacuum
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Electric vacuum pump
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Hydraulic brake assist
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Electronic brake booster
The booster provides consistent braking performance while reducing driver effort.
It also supports several advanced safety systems, including:
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Anti-lock Braking System (ABS)
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Electronic Stability Control (ESC)
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Brake Assist System (BAS)
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Automatic Emergency Braking
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Adaptive Cruise Control
Why Is Brake Booster Performance Important?
The brake booster allows the driver to stop the vehicle using reasonable pedal effort.
If booster performance decreases:
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Brake pedal effort increases.
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Emergency braking becomes more difficult.
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Stopping distances may increase.
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Electronic safety systems may become less effective.
Symptoms of C0021
The severity of symptoms depends on the amount of brake assist that has been lost.
Brake Warning Light
The brake warning light may illuminate.
ABS Warning Light
Some vehicles also activate the ABS warning light.
Electronic Stability Control Warning
The ESC warning light may appear.
Hard Brake Pedal
One of the most common symptoms is a brake pedal that feels much harder than normal.
Increased Pedal Effort
The driver may need significantly more force to stop the vehicle.
Longer Stopping Distance
Reduced brake assist may increase braking distance, particularly during emergency braking.
Inconsistent Brake Feel
Drivers may notice:
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Hard pedal
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Delayed brake response
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Uneven brake assist
Brake Assist Disabled
Emergency Brake Assist may stop functioning.
Driver Assistance Systems Disabled
Depending on the vehicle, the following systems may also become unavailable:
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Adaptive Cruise Control
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Automatic Emergency Braking
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Collision Mitigation Braking
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Hill Start Assist
Common Causes of C0021
Several mechanical and electrical issues may reduce brake booster performance.
Vacuum Leak
Vacuum-controlled brake boosters may suffer from:
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Cracked vacuum hoses
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Loose hose connections
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Damaged booster diaphragm
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Vacuum check valve failure
Vacuum leaks are among the most common causes.
Faulty Brake Booster
Internal booster failures may include:
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Torn diaphragm
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Mechanical wear
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Internal air leakage
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Seal failure
Electric Vacuum Pump Failure
Vehicles equipped with electric vacuum pumps may experience:
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Pump failure
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Low vacuum generation
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Relay problems
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Electrical supply issues
Brake Booster Solenoid Problems
A sticking or malfunctioning brake booster solenoid may prevent proper vacuum regulation.
Hydraulic Brake Booster Problems
Hydraulic brake assist systems may experience:
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Low hydraulic pressure
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Internal leakage
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Pressure regulator faults
Brake Pressure Sensor Problems
Incorrect pressure readings may cause the control module to interpret booster performance incorrectly.
Brake Pedal Position Sensor Problems
Incorrect pedal input information may also trigger booster performance faults.
ABS or Brake Control Module Problems
Although less common, the control module itself may develop:
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Internal faults
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Software problems
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Incorrect brake assist calculations
Vehicles Commonly Affected by C0021
C0021 may appear on many vehicles equipped with electronically monitored brake booster systems.
Examples include:
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Ford F-150
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Ford Explorer
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Chevrolet Silverado
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Chevrolet Tahoe
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GMC Sierra
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Dodge Ram
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Jeep Grand Cherokee
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Toyota Camry
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Toyota Highlander
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Honda Accord
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Honda CR-V
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Nissan Altima
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Hyundai Santa Fe
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Kia Sorento
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Volkswagen Passat
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Audi A6
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BMW 5 Series
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Mercedes-Benz E-Class
Brake booster designs vary considerably between manufacturers, so diagnostic procedures may differ.
How C0021 Is Diagnosed
Proper diagnosis requires inspection of both the brake booster and its supporting systems.
Read Diagnostic Trouble Codes
A professional diagnostic scanner retrieves:
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ABS codes
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Brake control module codes
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Stability control codes
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Freeze-frame information
Related brake system codes often help identify the root cause.
Monitor Live Brake Data
Technicians evaluate:
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Brake booster performance
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Brake pressure
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Booster vacuum
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Pedal force
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Pedal travel
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Brake assist response
Inspect the Brake Booster
The booster should be checked for:
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Vacuum retention
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Internal leakage
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Mechanical damage
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Proper operation
Inspect the Vacuum System
Technicians inspect:
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Vacuum hoses
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Check valve
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Vacuum pump output
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Hose connections
Inspect Electrical Components
Electrical inspection includes:
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Brake booster solenoid
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Pressure sensors
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Pedal sensors
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Wiring
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Connectors
Inspect the Charging System
Vehicles equipped with electric brake boosters require proper charging voltage.
Technicians verify:
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Battery condition
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Charging voltage
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Alternator performance
How to Fix C0021
The appropriate repair depends on the underlying cause.
Repair Vacuum Leaks
Vacuum leaks should be repaired by replacing:
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Vacuum hoses
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Check valves
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Damaged fittings
Replace the Brake Booster
If the booster has internal mechanical damage, replacement is usually necessary.
Replace the Electric Vacuum Pump
If the electric vacuum pump cannot generate sufficient vacuum, replacement may be required.
Replace the Brake Booster Solenoid
If testing confirms poor solenoid operation, replacement is recommended.
Repair Wiring Problems
Electrical repairs may include:
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Repair broken wires
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Replace damaged wiring
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Restore insulation
Replace Faulty Sensors
Brake pressure sensors or brake pedal position sensors should be replaced if they provide incorrect information.
Repair or Replace the ABS Module
If every other component tests correctly, the control module may require:
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Software updates
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Reprogramming
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Replacement
After repairs:
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Clear diagnostic trouble codes.
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Verify proper brake booster performance.
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Perform a road test.
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Confirm that C0021 does not return.
Can You Drive With C0021?
Driving with C0021 may still be possible, but it is not recommended.
Reduced brake booster performance means the driver may need much greater force on the brake pedal.
Emergency braking performance may be significantly reduced.
The vehicle should be inspected as soon as possible.
Is C0021 a Serious Code?
C0021 is generally considered a high severity brake system fault.
Even though the hydraulic braking system often continues functioning, reduced brake assist can greatly increase braking effort and stopping distance.
Prompt diagnosis and repair are strongly recommended.
How to Prevent C0021
Routine brake system maintenance can reduce the likelihood of brake booster performance problems.
Recommended maintenance includes:
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Inspect vacuum hoses regularly.
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Test vacuum pump operation.
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Replace worn brake booster components.
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Maintain the charging system.
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Inspect brake sensors during routine service.
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Address brake warning lights immediately.
Final Thoughts
The C0021 Brake Booster Performance (Subfault) code indicates that the brake control system has detected that the brake booster is no longer providing the expected level of brake assistance.
The most common causes include:
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Vacuum leaks
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Brake booster failure
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Electric vacuum pump problems
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Brake booster solenoid faults
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Brake pressure sensor problems
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Brake pedal position sensor faults
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ABS or brake control module failures
Because the brake booster plays a critical role in reducing pedal effort and maintaining safe braking performance, the entire brake assist system should be properly diagnosed and repaired to restore full braking capability and ensure safe vehicle operation.
