C0021 - Chassis Fault Code C0021 - ABS / Bromssystem (Hjulhastighetssensorer & Ventiler)
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System
Chassis (Brakes, Steering, Suspension)
Related Fault Codes
Chassis Fault Code C0001 - ABS / Bromssystem (Hjulhastighetssensorer & Ventiler)
Chassis Fault Code C0002 - ABS / Bromssystem (Hjulhastighetssensorer & Ventiler)
Chassis Fault Code C0003 - ABS / Bromssystem (Hjulhastighetssensorer & Ventiler)
Chassis Fault Code C0004 - ABS / Bromssystem (Hjulhastighetssensorer & Ventiler)
Technical Description
DTC C0021 indicates an electrical or performance fault in the Brake Booster Pressure/Vacuum Sensor circuit. The ABS/ESC module monitors the sensor's analog return voltage against internal plausibility thresholds (cross-referenced with engine MAP, throttle position, and auxiliary vacuum pump actuation). A circuit open, short to ground/reference, or mechanical loss of booster vacuum triggers this DTC, leading to disabled hydraulic brake assist, start-stop suppression, or degraded ESP functionality.
Possible Causes
1. Mekaniskt vakuumläckage (sprucken vakuumslang, otät backventil eller läckande bromsservomembran)
2. Defekt bromsservotryckgivare (internt piezoresistivt elementfel eller kontaktkorrosion)
3. Kablagefel (avbrott på 5V-referens eller signalledning, eller kortslutning till jord)
4. Defekt mekanisk vakuumpump (diesel/direktinsprutad bensin) eller felande elektrisk hjälpvakuumpump/relä
5. Hårdvarufel eller internt kontaktfel i ABS/ESP-styrdonet
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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