What it means
The brake control module detects that the brake booster is not providing the expected assistance. It sets the code when monitored booster performance falls outside the module's calibrated limits.
How it works
The brake booster reduces the force needed at the pedal by adding assistance to the driver’s braking effort. The brake control module monitors booster performance and stores C0021 when the assistance it detects falls outside its calibrated limits.
Common causes
Most likely first.
- Vacuum leak or damaged hose
- Failing brake booster
- Low engine vacuum
- Faulty vacuum or pressure sensor
- Wiring or connector fault
Symptoms
- Hard brake pedal
- Increased pedal effort
- Longer stopping distance
- ABS or brake warning light
How to diagnose it
- Read brake-control and engine codes with freeze-frame data to see whether the fault coincides with low engine vacuum or another system problem.
- Inspect the booster vacuum hose and connections for splits, looseness, collapse, or leaks.
- Check engine vacuum and the booster’s vacuum or pressure sensor readings against the manufacturer’s specification.
- Inspect the sensor and booster wiring and connectors for damage, poor contact, or corrosion.
- Test the booster’s assistance and check for leaks using the vehicle manufacturer’s procedure.
- If vacuum supply, sensor readings, and wiring are correct, evaluate the brake booster and control module.
Common fixes
- Reconnect or replace a leaking or damaged booster vacuum hose.
- Repair a wiring or connector fault at the booster sensor.
- Replace a faulty vacuum or pressure sensor if testing confirms it.
- Correct an engine condition that is causing low vacuum.
- Replace a failed brake booster if it cannot provide the specified assistance.
Can I keep driving?
A hard pedal or increased stopping distance can make the vehicle difficult to stop, so avoid driving if pedal effort is unusually high or braking performance is reduced. Have the vehicle checked promptly and arrange assistance if it cannot be stopped safely.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.