Fault Family System: Chassis (C-Codes) 6 SAE Codes

Brake Booster Solenoid Circuit

Diagnostic & Technical Explanation

Code C0021 (Brake Booster Performance) sets when the module detects that the vacuum or hydraulic brake booster is not building or holding the assist level its pressure model expects, or the booster's control solenoid shows an open circuit, an out-of-range voltage, or a driver-stage fault. Typical causes are a leaking booster diaphragm, a cracked vacuum hose or check valve, or a failed solenoid coil that cannot be commanded to its seated or unseated position. A weak booster shows up as unusually high, inconsistent brake pedal effort rather than a full brake failure.

These 6 diagnostic trouble codes share one detection mechanism and one repair path, which is why MechanicDB groups them into a single authored fault family rather than repeating the same explanation 6 times.

Codes in this family (6)

Code	Definition	Repair coverage
C0021	Brake Booster Performance	4 ranked fixes
C0022	Brake Booster Solenoid	4 ranked fixes
C0566	Brake Booster Solenoid Circuit/Open	4 ranked fixes
C0567	Brake Booster Solenoid Circuit Low	4 ranked fixes
C0568	Brake Booster Solenoid Circuit High	4 ranked fixes
C0569	Brake Booster Solenoid Circuit Performance	4 ranked fixes

What this page does not show

This page publishes the fault mechanism and the code index. The licensed dataset adds, for every one of the 15,886 codes, the full authored repair set (56,729 procedures in total) with the fields deliberately withheld here:

Procedures 2..n after the first-listed procedure
Aftermarket parts cost ranges in USD (est_parts_cost_min/max_usd)
Professional labor-hour estimates (est_labor_hours)
Step-by-step diagnostic and repair instructions
Replacement-part mappings and catalog lookups
6,637 manufacturer-specific OEM codes across 32 makes ($149 tier)

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