REFERENCE · LIVE
A5 Brake Symbol Key
Every brake-system dash symbol, what it means, and the DTC family it usually rolls with. Customer points at a light — you point at this card and know exactly which subsystem you're walking into.
Color legend
Dash Symbols
Brake Warning (!)
Red. Low fluid, parking brake set, or hydraulic pressure differential. Stop driving until verified.
DTC family: C012X / C013X (master cyl)
ABS Warning
Amber. Anti-lock disabled — base brakes still work. Most common cause: wheel speed sensor circuit.
DTC family: C003X – C006X
Traction Control / TCS
Amber if continuous = system fault. Flashes during normal activation. Car with skid marks symbol.
DTC family: C006X – C008X
ESC / Stability Control
Amber. Often paired with TCS light. Yaw, steering angle, or lateral accel sensor fault. Limp mode on stability.
DTC family: C015X – C016X
Parking Brake (P)
Red. On = parking brake applied. Stays on with rolling = release switch stuck or cable jammed.
DTC family: C055X (EPB)
Electric Parking Brake Fault
Amber. EPB motor or module error. Service-mode required before pad replacement.
DTC family: C055X / C056X
BAS / Brake Assist
Amber. Brake assist fault, often triggered alongside ABS. Booster pressure sensor or pedal switch related.
DTC family: C012X (booster)
Brake Pad Wear
Amber. Electronic pad-wear sensor circuit complete (worn through, or harness broken). Verify visually before condemning sensor.
DTC family: C040X – C041X
Auto Hold / Brake Hold (A)
Green/white. Driver-activated hold feature engaged. Common on EVs and modern automatics.
Status only — no DTC unless paired with EPB fault
AWS / Adaptive Cruise Brake Fault
Amber. ADAS calibration or front radar issue prevents auto-brake function. Often after windshield work.
DTC family: U-codes (network) + C12XX
RAB / Rear Auto Braking
Amber. Reverse auto-brake disabled — usually rear sensor fault, dirty bumper, or tow hitch interference.
DTC family: B-codes + ECM disables
General Warning Triangle
Red (some platforms amber). Pairs with text — "Brake System Service" or "Stability Control Off." Scan codes immediately.
Text on cluster identifies system
C-Code DTC Families
"C" codes are chassis codes — most brake-system DTCs live here. Generic SAE ranges are below; manufacturer-specific codes (1xxx) override these definitions per platform.
Wheel Speed Sensor — General Circuit
Most common ABS family. C0035 = LF, C0040 = RF, C0045 = LR, C0050 = RR (SAE generic). Suspect tone ring first on low-speed-only faults.
Wheel Speed Sensor — Signal Performance
Erratic or noisy speed signal. Often paired with stability-light intermittents on rough roads.
Hydraulic Pump / Motor Circuit
ABS pump motor fault. Will set when pump can't reach target pressure or run-time exceeds threshold.
Solenoid Valve Circuits (Apply / Release)
HCU isolation and dump valves. Usually internal modulator failure when these set with no harness damage.
Brake Booster / Master Cylinder Pressure
Booster vacuum or hydroboost pressure outside spec. Test power source before condemning unit.
Brake Fluid Level / Pressure Differential
Float switch or differential pressure circuit. Often paired with red brake light.
Yaw Rate / Lateral Accel Sensor
Stability sensor calibration or wiring. Most platforms require zero-point reset after replacement.
Steering Angle Sensor
SAS calibration loss after alignment, suspension work, or battery disconnect. Re-zero with scan tool.
Brake Pad Wear Sensor Circuit
Electronic wear sensor open or shorted. Inspect pads visually before replacing sensor.
Electric Parking Brake (EPB) — Motor / Module
EPB caliper motor fault. Use scan-tool service mode before pad work — never C-clamp.
Network Communication (CAN bus)
U0121 = ABS module lost comm. ADAS, EPB, and brake-by-wire depend on bus health. Bus diag first when multiple modules report missing.
BAY-FLOOR RULES
Quick decisions when the light is on
- Red brake light + low fluid: stop driving conversation immediately. Pressure differential or hydraulic leak.
- ABS amber alone: base brakes still work. Pull C-codes and start at wheel speed sensors.
- ABS + TCS + ESC all amber: almost always a wheel speed or steering angle sensor — single root cause cascades to all three.
- EPB amber: never push the rear piston in with a C-clamp. Use scan-tool service mode.
- Multiple U-codes + brake warnings: diagnose the bus before chasing individual modules.
- Brake pad wear lamp on with new pads: reset the sensor connector or replace the wear sensor — most are single-use.