Car systems
Brakes, ABS and ESC: how they work and what fails
The brakes turn the pressure on the pedal into force on the rotors and drums; ABS keeps the wheels from locking and stability control (ESC) brakes single wheels so the car does not skid. The red brake warning light, a pedal that sinks or a metallic noise when braking are reasons to stop and not drive on.
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How it works
The pedal pushes the master cylinder and the fluid carries the pressure to the calipers, which press the pads against the rotors; the brake booster multiplies the force of your foot. The fluid absorbs moisture over the years, which is why it is changed on the maintenance schedule.
The ABS module compares the speed of each wheel with its sensors and, if one is about to lock, releases and restores the pressure several times per second with its hydraulic unit. Stability control adds yaw, lateral acceleration and steering angle sensors. The electric parking brake has motors on the rear calipers and its own control unit.
The most common faults
- Worn pads and rotors: the squeal of the wear indicator, a metallic noise or a vibrating steering wheel when braking.
- Wheel speed sensors: dirt, a broken wire or a damaged tone ring: the ABS and ESC lights come on and the car brakes without electronic help.
- Brake fluid: a low level from wear or a leak, or old fluid: a soft pedal or one that sinks.
- Brake pedal switch or sensor: the brake lights misbehave and cruise control or stability control switch off.
- Electric parking brake: caliper motor, wiring or a weak battery: it does not release or apply and turns on its light.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
ABS or ESP warning light on: what it means and what to do
See a garage soon
Red brake warning light on: what it means and what to do
Stop as soon as it is safe
Noise when braking: what a squeal or a metallic grinding means
See a garage soon
Vibration when braking: why the steering wheel or pedal shakes
See a garage soon
Soft, long or sinking brake pedal: what to do
Stop as soon as it is safe
Electric parking brake warning light on: what to do
See a garage soon
Car pulls to one side: causes and what to do
See a garage soon
Burning smell in the car: what it could be and what to do
Stop as soon as it is safe
The most relevant trouble codes
- C0035Right front wheel speed sensor supply: malfunction
- C0040Brake pedal switch A: malfunction
- C0045Brake pressure sensor B: malfunction
- C0031Left front wheel speed sensor: malfunction
- C0034Right front wheel speed sensor: malfunction
- C0020ABS pump motor control: malfunction
- C0049Brake fluid level: malfunction
- C004ABrake lining wear sensor: malfunction
- C0609Park brake switch, circuit: malfunction
- C060ELeft park brake actuator: performance or stuck off
- C0613Right park brake actuator: performance or stuck off
- C0616Park brake control module, control circuit: performance
- U0121Lost communication with anti-lock brake system (ABS) control module A
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
Questions and articles about this system
- What do U codes and lost communication (U0100) mean?
- Can I diagnose my car myself?
- How often should you run diagnostics on your car?
Learn more
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
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Frequently asked questions
Can I drive with the ABS light on?
The normal brakes usually still work, but without ABS or stability control: leave more distance and have the car checked soon. If the red brake light comes on too, stop.
How often should brake fluid be changed?
The manufacturer says; every two years is common, because the fluid absorbs moisture and its boiling point drops.
Can an OBD reader read the ABS?
The generic chassis (C) codes the car publishes, yes. Many brands also use their own codes in the ABS module, which need their tool or a compatible one.