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Home / DTC / C0021 — Brake Booster Performance (Subfault)

C0021 — Brake Booster Performance (Subfault)

Detailed page for trouble code C0021.

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Code

C0021

HYUNDAI C — Chassis

Brake Booster Performance (Subfault)

Brand: HYUNDAI
Type: C — Chassis
AI status
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ready
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Page language: EN

Causes

  • Vacuum leak in hose, fittings, or intake manifold
  • Failed brake booster (diaphragm leak or internal failure)
  • Faulty one-way check valve or non-return valve
  • Defective vacuum pump (on vehicles with engine-driven or electric pumps)
  • Low brake fluid or master cylinder issues affecting booster operation
  • Faulty booster pressure/vacuum sensor or switch

Symptoms

  • Increased brake pedal effort (harder pedal)
  • Longer brake pedal travel before braking effect
  • Delayed or reduced braking assistance, especially at idle
  • Brake warning light or ABS/ESC warnings illuminated
  • Hissing noise from footwell area (vacuum leak)
  • Engine rough idle or stalling (large vacuum leak)

What to check

  • Read and record all stored codes and freeze-frame data from all modules (ABS, BCM, ECU)
  • Capture live data: booster vacuum/pressure sensor values, brake pedal position, master cylinder pressure, pump current (if equipped) and battery voltage
  • Visual inspection of vacuum lines, check valve, vacuum pump and connectors for damage or disconnection
  • Check brake fluid level and inspect master cylinder for leaks
  • Listen for hissing at booster with engine running and during pedal application
  • Perform a vacuum hold test on the booster and hoses with a gauge or hand pump

Signal parameters

  • Brake booster vacuum (kPa or inHg)
  • Brake booster pressure sensor voltage or pressure reading
  • Brake pedal position (%) or pedal travel (mm)
  • Master cylinder pressure (bar or psi)
  • Vacuum pump current draw (A) or duty cycle (if electric)
  • Engine intake vacuum (kPa/inHg)

Diagnostic algorithm

  1. Connect a full-function scan tool, read/record all codes and freeze-frame data from ABS/Brake/ECU modules. Note context (engine speed, pedal position).
  2. Perform a visual inspection of vacuum hoses, check valve, vacuum pump and booster for damage, routing issues, or disconnections. Check brake fluid level and master cylinder for signs of leakage into booster.
  3. With engine off, inspect the one-way check valve for contamination and proper seating. Manually apply vacuum to test for hold. Replace if suspect.
  4. Start engine and measure vacuum at booster and intake. Compare readings to manufacturer specifications. Observe vacuum while applying pedal and during idle/acceleration. Look for drops or inability to reach target vacuum.
  5. If equipped, test electric vacuum pump output and verify pump operation, current draw and vacuum generation. Replace or repair pump if below spec.
  6. Use a vacuum gauge to perform a booster vacuum-hold test (engine off) to check diaphragm integrity. Loss of vacuum indicates internal booster failure.
  7. Check electrical connectors and wiring to any booster sensors/solenoids for continuity, corrosion and proper reference voltage/signals. Repair as required.
  8. If tests point to booster/mechanical failure, remove and bench-test or replace the brake booster and associated check valve. Replace master cylinder if contaminated or leaking into booster.
  9. Clear codes and perform a cautious stationary and road test to verify proper brake assistance and absence of the C0021 code. Re-scan all modules for reappearance or related faults.

Likely causes

  • Vacuum hose disconnected or cracked near booster
  • Failed booster check valve allowing loss of vacuum
  • Intermittent vacuum pump operation or low pump output
  • Corroded connector or broken wire to booster sensor/solenoid
  • Master cylinder seals leaking into booster

Fault status

⚠️ Status
Brake Booster Performance (Subfault) — the brake control system detected degraded or abnormal brake booster performance. Inspect vacuum/hydraulic supply, check valve, booster integrity, pump (if fitted), sensors, wiring, and related module communications.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

C0020 C0022 C0035 C0040 U0121
371

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