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Home / DTC / C0021 — Brake Booster Performance

C0021 — Brake Booster Performance

Detailed page for trouble code C0021.

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Code

C0021

Generic C — Chassis

Brake Booster Performance

Brand: Generic
Type: C — Chassis
Views: UK: 26 EN: 95 RU: 30
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Vacuum leak in booster supply hose or intake manifold
  • Failed one‑way check valve (vacuum check valve)
  • Failed brake booster diaphragm or internal leakage
  • Faulty vacuum pump (on vehicles with pump-assisted systems)
  • Faulty electronic brake booster (eBooster) or its internal motor/controller
  • Bad vacuum reservoir or collapsed hose

Symptoms

  • Increased brake pedal effort (harder pedal) or reduced power assist
  • Longer stopping distances or reduced braking performance
  • Brake warning light or ABS/warning lamp illuminated
  • Hissing noise from engine bay near booster (vacuum leak)
  • Excessive brake pedal travel or inconsistent pedal feel
  • Possible engine idle changes or rough idle (if vacuum leak into intake)

What to check

  • Read fault code(s) and freeze frame with a scan tool; record status and any related codes
  • Check brake fluid level and general brake system condition
  • Visually inspect vacuum hoses, booster-to-intake hose, check valve and vacuum reservoir for damage, crimps or disconnection
  • Listen for audible hissing at the booster area with engine running (use caution)
  • Check fuses and relays for eBooster/vacuum pump circuits (if equipped)
  • Inspect electrical connectors at booster, vacuum sensor, pump and control module for corrosion or looseness

Signal parameters

  • Vacuum at booster/manifold at idle: typically ~15–22 inHg (approx. 50–75 kPa) — vehicle-specific; lower vacuum indicates leak or pump failure
  • Vacuum sensor output (if fitted): typically a proportional voltage signal (example range 0.5–4.5 V) — compare to manufacturer spec
  • eBooster motor supply: battery voltage at connector ~12 V (engine off/ignition on) and full system voltage under load
  • Pump current draw/pulse duty cycle: should be within manufacturer limits (use service data)
  • ABS/Brake module communications: CAN bus messages present and not showing errors

Diagnostic algorithm

  1. Record DTC, freeze‑frame and any related codes using a scan tool. Clear codes and re‑check to confirm repeatability.
  2. Perform a visual inspection: vacuum hoses, check valve orientation, reservoir, booster body and connectors. Repair or secure any obvious issues.
  3. With engine off, inspect the booster hose and one‑way check valve; verify it holds vacuum with a hand vacuum pump/gauge where applicable.
  4. Start engine and measure vacuum at the booster/manifold with a vacuum gauge. Compare readings to vehicle specification. Note changes with engine speed.
  5. If vacuum is low or fluctuating, isolate source: disconnect booster hose and test manifold vacuum; test vacuum pump output (if fitted) and inspect pump drive and plumbing.
  6. If vacuum supply is good, perform functional test of the booster: with engine running, apply steady brake pedal pressure and shut off engine — pedal should slowly sink if booster holds vacuum. (Perform safely with vehicle secured and chocked.)
  7. For eBooster systems: verify fuses/relays, battery/chassis ground, measure motor supply voltage and control signals, check connector continuity; consult manufacturer for motor current and PWM values.
  8. Inspect wiring and CAN communication to ABS/BCM for errors (U‑codes). Repair wiring or module issues as required.
  9. If booster or eBooster hardware or vacuum pump is suspected faulty after tests, replace the failed component and clear codes. Relearn procedures may be required for some systems; verify repair by road test and re‑scanning.
  10. If problem persists, consult vehicle‑specific service manual and manufacturer‑level diagnostics.

Likely causes

  • Leaking or disconnected vacuum hose or failed check valve
  • Failed booster diaphragm or loss of internal vacuum
  • Vacuum pump failure (on pump-equipped vehicles)
  • Electrical fault at eBooster (blown fuse, relay, poor battery supply, damaged connector)

Fault status

⚠️ Status
Brake Booster Performance Fault — Control module detected inadequate or abnormal brake booster assist (vacuum or electronic). Inspect vacuum supply, booster, eBooster and related electrical/communication circuits.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

C0020 C0025 C0035 U0121 P0236
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Code

C0021

HYUNDAI C — Chassis

Brake Booster Performance (Subfault)

Brand: HYUNDAI
Type: C — Chassis
Views: UK: 14 EN: 78 RU: 24
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Vacuum leak in booster supply hose or intake manifold
  • Failed one‑way check valve (vacuum check valve)
  • Failed brake booster diaphragm or internal leakage
  • Faulty vacuum pump (on vehicles with pump-assisted systems)
  • Faulty electronic brake booster (eBooster) or its internal motor/controller
  • Bad vacuum reservoir or collapsed hose

Symptoms

  • Increased brake pedal effort (harder pedal) or reduced power assist
  • Longer stopping distances or reduced braking performance
  • Brake warning light or ABS/warning lamp illuminated
  • Hissing noise from engine bay near booster (vacuum leak)
  • Excessive brake pedal travel or inconsistent pedal feel
  • Possible engine idle changes or rough idle (if vacuum leak into intake)

What to check

  • Read fault code(s) and freeze frame with a scan tool; record status and any related codes
  • Check brake fluid level and general brake system condition
  • Visually inspect vacuum hoses, booster-to-intake hose, check valve and vacuum reservoir for damage, crimps or disconnection
  • Listen for audible hissing at the booster area with engine running (use caution)
  • Check fuses and relays for eBooster/vacuum pump circuits (if equipped)
  • Inspect electrical connectors at booster, vacuum sensor, pump and control module for corrosion or looseness

Signal parameters

  • Vacuum at booster/manifold at idle: typically ~15–22 inHg (approx. 50–75 kPa) — vehicle-specific; lower vacuum indicates leak or pump failure
  • Vacuum sensor output (if fitted): typically a proportional voltage signal (example range 0.5–4.5 V) — compare to manufacturer spec
  • eBooster motor supply: battery voltage at connector ~12 V (engine off/ignition on) and full system voltage under load
  • Pump current draw/pulse duty cycle: should be within manufacturer limits (use service data)
  • ABS/Brake module communications: CAN bus messages present and not showing errors

Diagnostic algorithm

  1. Record DTC, freeze‑frame and any related codes using a scan tool. Clear codes and re‑check to confirm repeatability.
  2. Perform a visual inspection: vacuum hoses, check valve orientation, reservoir, booster body and connectors. Repair or secure any obvious issues.
  3. With engine off, inspect the booster hose and one‑way check valve; verify it holds vacuum with a hand vacuum pump/gauge where applicable.
  4. Start engine and measure vacuum at the booster/manifold with a vacuum gauge. Compare readings to vehicle specification. Note changes with engine speed.
  5. If vacuum is low or fluctuating, isolate source: disconnect booster hose and test manifold vacuum; test vacuum pump output (if fitted) and inspect pump drive and plumbing.
  6. If vacuum supply is good, perform functional test of the booster: with engine running, apply steady brake pedal pressure and shut off engine — pedal should slowly sink if booster holds vacuum. (Perform safely with vehicle secured and chocked.)
  7. For eBooster systems: verify fuses/relays, battery/chassis ground, measure motor supply voltage and control signals, check connector continuity; consult manufacturer for motor current and PWM values.
  8. Inspect wiring and CAN communication to ABS/BCM for errors (U‑codes). Repair wiring or module issues as required.
  9. If booster or eBooster hardware or vacuum pump is suspected faulty after tests, replace the failed component and clear codes. Relearn procedures may be required for some systems; verify repair by road test and re‑scanning.
  10. If problem persists, consult vehicle‑specific service manual and manufacturer‑level diagnostics.

Likely causes

  • Leaking or disconnected vacuum hose or failed check valve
  • Failed booster diaphragm or loss of internal vacuum
  • Vacuum pump failure (on pump-equipped vehicles)
  • Electrical fault at eBooster (blown fuse, relay, poor battery supply, damaged connector)

Fault status

⚠️ Status
Brake Booster Performance Fault — Control module detected inadequate or abnormal brake booster assist (vacuum or electronic). Inspect vacuum supply, booster, eBooster and related electrical/communication circuits.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

C0020 C0025 C0035 U0121 P0236
Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

C0021

LAND ROVER C — Chassis

Performance of brake accelerator

Brand: LAND ROVER
Type: C — Chassis
Views: UK: 9 EN: 68 RU: 17
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Vacuum leak in booster supply hose or intake manifold
  • Failed one‑way check valve (vacuum check valve)
  • Failed brake booster diaphragm or internal leakage
  • Faulty vacuum pump (on vehicles with pump-assisted systems)
  • Faulty electronic brake booster (eBooster) or its internal motor/controller
  • Bad vacuum reservoir or collapsed hose

Symptoms

  • Increased brake pedal effort (harder pedal) or reduced power assist
  • Longer stopping distances or reduced braking performance
  • Brake warning light or ABS/warning lamp illuminated
  • Hissing noise from engine bay near booster (vacuum leak)
  • Excessive brake pedal travel or inconsistent pedal feel
  • Possible engine idle changes or rough idle (if vacuum leak into intake)

What to check

  • Read fault code(s) and freeze frame with a scan tool; record status and any related codes
  • Check brake fluid level and general brake system condition
  • Visually inspect vacuum hoses, booster-to-intake hose, check valve and vacuum reservoir for damage, crimps or disconnection
  • Listen for audible hissing at the booster area with engine running (use caution)
  • Check fuses and relays for eBooster/vacuum pump circuits (if equipped)
  • Inspect electrical connectors at booster, vacuum sensor, pump and control module for corrosion or looseness

Signal parameters

  • Vacuum at booster/manifold at idle: typically ~15–22 inHg (approx. 50–75 kPa) — vehicle-specific; lower vacuum indicates leak or pump failure
  • Vacuum sensor output (if fitted): typically a proportional voltage signal (example range 0.5–4.5 V) — compare to manufacturer spec
  • eBooster motor supply: battery voltage at connector ~12 V (engine off/ignition on) and full system voltage under load
  • Pump current draw/pulse duty cycle: should be within manufacturer limits (use service data)
  • ABS/Brake module communications: CAN bus messages present and not showing errors

Diagnostic algorithm

  1. Record DTC, freeze‑frame and any related codes using a scan tool. Clear codes and re‑check to confirm repeatability.
  2. Perform a visual inspection: vacuum hoses, check valve orientation, reservoir, booster body and connectors. Repair or secure any obvious issues.
  3. With engine off, inspect the booster hose and one‑way check valve; verify it holds vacuum with a hand vacuum pump/gauge where applicable.
  4. Start engine and measure vacuum at the booster/manifold with a vacuum gauge. Compare readings to vehicle specification. Note changes with engine speed.
  5. If vacuum is low or fluctuating, isolate source: disconnect booster hose and test manifold vacuum; test vacuum pump output (if fitted) and inspect pump drive and plumbing.
  6. If vacuum supply is good, perform functional test of the booster: with engine running, apply steady brake pedal pressure and shut off engine — pedal should slowly sink if booster holds vacuum. (Perform safely with vehicle secured and chocked.)
  7. For eBooster systems: verify fuses/relays, battery/chassis ground, measure motor supply voltage and control signals, check connector continuity; consult manufacturer for motor current and PWM values.
  8. Inspect wiring and CAN communication to ABS/BCM for errors (U‑codes). Repair wiring or module issues as required.
  9. If booster or eBooster hardware or vacuum pump is suspected faulty after tests, replace the failed component and clear codes. Relearn procedures may be required for some systems; verify repair by road test and re‑scanning.
  10. If problem persists, consult vehicle‑specific service manual and manufacturer‑level diagnostics.

Likely causes

  • Leaking or disconnected vacuum hose or failed check valve
  • Failed booster diaphragm or loss of internal vacuum
  • Vacuum pump failure (on pump-equipped vehicles)
  • Electrical fault at eBooster (blown fuse, relay, poor battery supply, damaged connector)

Fault status

⚠️ Status
Brake Booster Performance Fault — Control module detected inadequate or abnormal brake booster assist (vacuum or electronic). Inspect vacuum supply, booster, eBooster and related electrical/communication circuits.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

C0020 C0025 C0035 U0121 P0236
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Short description
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Land Rover Defender 300Tdi — Workshop Manual (1996 model year)

Workshop Manual
Defender 300Tdi Years: 1996 Manual in English 7.5 MB
Short description

Official workshop manual for the Land Rover Defender 300Tdi (from 1996 model year). Contains specifications, adjustment, fault diagnosis and step-by-step repair and overhaul procedures for engine, transmission, axles, suspension, brakes, electrical and body. Intended for dealer workshops and trained technicians.

199,00 UAH
Contents
Key sections:
  • 01 INTRODUCTION
  • 04 GENERAL SPECIFICATION DATA
  • 05 ENGINE TUNING DATA
  • 07 GENERAL FITTING REMINDERS
  • 09 LUBRICANTS, FLUIDS AND CAPACITIES
  • 10 MAINTENANCE
  • 12 ENGINE Tdi
  • - Description and operation
  • - Fault diagnosis
  • - Adjustment
  • - Repair and overhaul procedures
  • 19 FUEL SYSTEM Tdi
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Workshop Manual
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Short description

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199,00 UAH
Contents
Key sections:
  • 01 - INTRODUCTION
  • - Introduction
  • - Dimensions
  • - References
  • - Repairs and replacements
  • - Poisonous substances
  • - Fuel handling precautions
  • - Synthetic rubber
  • - Recommended sealants
  • - Used engine oil precautions
  • - Accessories and conversions
  • - Wheels and tyres
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Land Rover Range Rover — Electrical Library (LRL 0453ENG, 2002)

Workshop Manual
Manual in English Pages: 494 7.1 MB
Short description

Land Rover Range Rover Electrical Library (LRL 0453ENG). Comprehensive manual covering fuse details, earth points, system descriptions, diagnostics and connector pin-outs for electrical troubleshooting and repair. Intended for technicians and service workshops.

199,00 UAH
Contents
Key sections:
  • 1 INTRODUCTION
  • 1.1 About this document
  • 1.2 Battery voltage
  • 1.3 Electrical precautions
  • 1.4 Battery disconnecting / charging
  • 1.5 Disciplines / greases
  • 1.6 Abbreviations
  • 1.7 HeVAC, sensors abbreviations
  • 1.8 How to use this document
  • 1.9 Connector detail format
  • 1.10 Fault diagnosis
  • 1.11 Wire colour codes
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