EN

Severity: High Code type: Generic (all makes) ✱

C0021 Code: Brake Booster Performance

The brake module thinks the power brake booster isn't giving the assist it should. Check pedal feel first.

Unverified: this manufacturer-specific definition comes from a single source or varies by model. Confirm it with a brand-specific scan tool.

Safety first · Consult a professional

  • Braking system affected: ABS may be disabled. Drive carefully and have it checked promptly.

At a glance

Severity: 4/5 – High4/5

Severity: High

Meaning
Brake Booster Performance
System
Brakes, ABS & Chassis · Chassis
Safe to drive?
With caution
Typical repair cost
$30–$3,000
DIY difficulty
Moderate

C0021 is the SAE name for a brake booster performance fault. The booster multiplies your foot force, using engine vacuum, a separate vacuum pump (diesels, turbocharged direct-injection and hybrid engines) or an electric/electro-hydraulic unit. A pressure sensor on the booster or a motor-position feedback tells the brake module how much assist is available; when that stays too low for too long after start-up, C0021 is stored and a brake or ABS warning appears. Ford and Chrysler-group vehicles use this definition, often with suffixes such as -02, -64 or -77; older proprietary ABS systems may assign C0021 a different meaning, so confirm the text shown by your scan tool.

How to read C0021

How to read C0021C10203214
  1. C Chassis
  2. 0 Generic (SAE)
  3. 0 Brakes, ABS & Chassis
  4. 21 Specific fault

Symptoms

  • Brake pedal hard or high, needing much more force to stop
  • Brake warning light and ABS or stability light on
  • Hissing noise under the dash when pressing the pedal
  • Engine idle rough or high because of a vacuum leak
  • Message such as 'brake assist reduced' or 'service brake booster'
  • Warning shows only in very cold weather or right after a cold start

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Cracked or disconnected vacuum hose, or failed one-way check valve at the booster 28%
  2. Weak or failed vacuum pump (engine-driven on diesels/turbo engines, electric on hybrids) 22%
  3. Leaking booster diaphragm or internal valve 18%
  4. Faulty booster vacuum/pressure sensor or its wiring and connector 16%
  5. Low engine vacuum from an intake leak, or software/calibration issue in the brake module 10%
  6. Electric brake booster or its control unit failing on vehicles without a vacuum booster 6%

Diagnosis step by step

Diagnostic flow: C00211Check pedal feel and boosterfunction2Read the brake module data3Inspect hoses and the check valve4Measure vacuum at the booster5Test the booster holds vacuum6Check the sensor circuit

  1. Check pedal feel and booster function

    With the engine off, pump the pedal five or six times to use up stored vacuum, then hold light pressure and start the engine. The pedal should sink slightly as assist arrives; if it stays rock-hard, the booster isn't getting vacuum or isn't working.

  2. Read the brake module data

    With an ABS-capable scan tool, read C0021 with its suffix and watch the booster vacuum or pressure value. A healthy vacuum booster typically sees about 17-21 inHg at warm idle on a gasoline engine; a reading stuck near zero with a good pedal points to the sensor instead of the booster.

    Tools: OBD-II scanner

  3. Inspect hoses and the check valve

    Follow the hose from the booster to the intake manifold or vacuum pump. Look for cracks, oil-softened rubber and loose clamps, and blow through the check valve: air should pass only toward the vacuum source.

    Tools: Flashlight, Pliers

  4. Measure vacuum at the booster

    Tee a vacuum gauge into the booster hose. At idle, gasoline engines should hold roughly 17-21 inHg and a diesel vacuum pump about 24-27 inHg; a low or slowly rising reading means a weak pump or restriction upstream.

    Tools: Vacuum gauge

  5. Test the booster holds vacuum

    Run the engine for a minute, shut it off and wait about 90 seconds. You should still get two or three assisted pedal applications; if the first press is already hard, the diaphragm or check valve is leaking.

  6. Check the sensor circuit

    Most booster sensors use a 5 V reference, a ground and a signal wire. Confirm about 5 V reference and a good ground at the connector, and compare the signal voltage change when the engine starts with the specification for your vehicle.

    Tools: Multimeter

Where the parts are

EVAP system: gas cap, canister, purge and vent valves – C0021123456
EVAP system: gas cap, canister, purge and vent valves
  1. 1Gas cap
  2. 2Charcoal canister
  3. 3Purge valve
  4. 4Vent valve
  5. 5Fuel pump
  6. 6Intake manifold

Repair options & costs

RepairDIY?Estimated cost
Replace vacuum hose and check valve Yes $30–$180
Replace booster vacuum sensor Yes $80–$300
Replace mechanical or electric vacuum pump No $350–$1,000
Replace brake booster No $450–$1,200
Replace electric brake booster unit No $1,200–$3,000

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Ford

Ford lists C0021 with suffix -77 ('commanded position not reachable') on several models; it tends to appear in severe cold, and Ford treats it as informational when the pedal feels normal and no other brake warnings are present.

Field data

Chevrolet 4 Silverado 1500
Dodge 3 Dart, Durango, Grand Caravan Cargo
Ram 2 2500

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Emissions / smog test impact

Depends

C0021 has no effect on an OBD emissions test, but a lit brake warning light or poor pedal assist will fail a state safety inspection where one is required.

Frequently asked questions

Are my brakes going to fail completely?

No. The hydraulic brakes still work without the booster, but you may need two to three times the pedal force to stop, which is dangerous in an emergency. That's why the pedal test decides whether you should drive at all.

Why does C0021 come up on a turbo or diesel engine?

Those engines produce little or no manifold vacuum, so the booster depends on a mechanical or electric vacuum pump. When that pump wears or its hose leaks, assist drops and the module stores C0021.

Can a vacuum leak cause both C0021 and a lean engine code?

Yes. A split booster hose or leaking diaphragm lets unmetered air into the intake, so C0021 often shows up next to P0171 or a high idle. Fixing the leak cures both.

Does C0021 always mean the same thing?

It's the SAE definition for booster performance on newer vehicles, but some older ABS systems used the number for other faults. Always read the description your scan tool shows for your exact vehicle.

Sources