By Core Brake Parts Tech Team — Circleville, OH warehouse
An ABS light on a 2012-2016 Audi A5 means the car's anti-lock brake computer has found a fault and switched ABS off, and with it the stability and traction control that use the same wheel speed signals. Your normal brakes still work, but the wheels can lock in a hard stop. The most common causes are a failed or damaged wheel speed sensor or its wiring, low brake fluid, and a wheel bearing or encoder problem, and the only reliable way to know which is to read the stored fault code.
How the ABS system works on the A5
This generation of A5 has four-wheel disc brakes, an electro-mechanical parking brake at the rear, and an ABS and stability control unit that watches each wheel individually. Audi's owner's manual for the A5 states that the ABS warning lamp monitors both the ABS and the electronic differential lock (EDL), the function that brakes a spinning wheel to send drive to the wheel with grip.
Every one of those functions depends on a wheel speed sensor at each corner. Bosch describes active wheel speed sensors as Hall-effect devices that read a rotating multipole magnetic encoder (or a toothed steel ring) and send digital pulses to the control unit, detecting speed almost down to a standstill. The ABS and the stability program (ESP/ESC) both run from that data. If one sensor's signal drops out or becomes implausible, the control unit cannot trust its calculation, so it shuts the functions off and lights the lamp. That is why a single bad sensor usually brings on the ABS light and the stability control warning together.
What the warning lights are telling you
| What you see | What it means | What to do |
|---|---|---|
| ABS lamp only | ABS (and usually ESC and traction control) is disabled; normal braking remains | Drive gently, leave more stopping distance, get the code read soon |
| ABS lamp plus red brake lamp | Audi's manual says ABS control may be out of action and the rear wheels can lock relatively easily when braking | Check the brake fluid level; drive carefully to a workshop |
| Red brake lamp with low fluid | Brake fluid in the reservoir is too low | Audi's manual says do not drive on; get professional help |
| Parking brake fault message | A fault in the electro-mechanical parking brake | Audi advises having it corrected by a workshop without delay |
| Brake pad wear symbol | The pads have reached the point where they need inspection | Have the pads inspected; this is separate from the ABS light |
The combination matters. An ABS lamp by itself is a fix-it-soon warning. An ABS lamp together with the red brake lamp is more serious, because the car has lost its ability to keep the rear wheels from locking under hard braking, and it can also point to a hydraulic problem such as low fluid.
Common causes of an ABS light on a 2012-2016 A5
1. Wheel speed sensor or its wiring
This is the first place to look. HELLA's technical guidance lists the typical sensor failure causes as broken wiring, internal short circuits, external damage, heavy contamination, increased wheel bearing clearance and mechanical damage to the encoder wheel. The sensors live behind each wheel in spray, salt and brake dust, and the harness flexes every time the suspension moves. HELLA even describes a case where a supply cable damaged by chafing caused a loose connection. On a car of this age, a cracked harness near the suspension or a corroded connector is just as likely as a dead sensor.
2. Wheel bearing play or a damaged encoder
The sensor reads a target that turns with the hub. If the bearing has developed play, or the encoder is damaged or contaminated with metal debris, the signal becomes erratic even though the sensor itself is fine. A replacement sensor will not cure this; the fault returns until the bearing or hub is fixed. Listen for a hum that changes with road speed, and check for play with the wheel off the ground.
3. Low brake fluid
Brake fluid drops slowly as pads wear, because the caliper pistons move out and the fluid follows them. A sudden drop means a leak. If the red brake lamp or a low fluid message comes on with the ABS lamp, check the reservoir before you do anything else. Do not simply top off a low reservoir on worn pads without finding out why it is low: fitting new pads pushes the pistons back and can overfill it.
4. Recent brake or suspension work
An ABS light that appears right after a brake job, a bearing replacement or a tire change is often a sensor that was pulled out, knocked, or left with its cable stretched or pinched. Look at the corner that was just worked on first.
5. ABS control unit or pump problems
Failures of the ABS hydraulic unit or its control module do happen, but they are much less common than sensor and wiring faults. Do not replace a module on a guess; confirm it with the fault codes and live data from a scan tool that can read Audi chassis codes.
Recalls
When we checked NHTSA's recall database for the 2013 through 2016 A5, the campaigns listed were for wiring and the engine cooling system, not the brakes or ABS. Always run your own VIN through NHTSA, because recall coverage is VIN-specific.
How to diagnose it step by step
- Check the brake fluid level. It takes seconds, and low fluid changes how urgent the problem is.
- Read the fault memory. A basic engine-code reader often cannot see ABS codes. Use a scan tool that reads the ABS module; the code will usually name the wheel.
- Watch live wheel speed data. Drive slowly with the scan tool connected. The failing wheel will read zero, drop out, or jump around while the other three agree.
- Inspect the sensor, harness and connector at that corner for chafing, corrosion and broken clips, as HELLA recommends, before buying parts.
- Check the wheel bearing for play and noise, since a loose bearing can cause the same code.
- Clear the code and road test. If it returns, the cause is still there.
The fix, and what to replace at the same time
Most A5 ABS lights end with a new wheel speed sensor, a harness repair, or a hub and bearing. Because the sensor sits right next to the brakes, it is a good time to look at the rest of that corner:
- Pads and rotors. If the pads are near the wear limit or the rotors are grooved, replace them together as an axle set. See our brake pads and brake rotors. For 2013 A5 Quattro owners, we carry the Hawk HPS 5.0 front brake pad set listed for that model.
- Pad wear sensor. If the pad wear warning has been triggered, replace the wear sensor along with the pads.
- Brake fluid. If the fluid is old or dark, a flush is cheap insurance for the ABS hydraulic unit. Use the fluid specification printed on the reservoir cap or in your owner's manual.
- Hardware and hoses. Replace worn clips with a brake hardware kit, and inspect the flexible brake hoses for cracks while the wheel is off.
Torque caliper bolts and wheel bolts to the values in the factory service information. For rear brake work, follow the factory procedure for the electro-mechanical parking brake, which generally means using a capable scan tool to open the parking brake before the rear pistons are retracted.
How hard is the job?
Reading codes needs a capable scan tool but no mechanical skill. Replacing a wheel speed sensor is usually a moderate job: wheel off, remove the retaining fastener, route the new cable into its clips, plug it in. Corrosion is the usual obstacle, because an old sensor can seize in the knuckle. A hub or bearing replacement is harder and often needs a press or special tools. Rear brake work on the A5 is where most home mechanics get caught out, because of the electronic parking brake. Without a scan tool that supports it, leave the rear brakes to a shop.
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Shop Brake Parts →Frequently Asked Questions
Is it safe to drive my Audi A5 with the ABS light on?
With the ABS lamp alone, normal braking still works, but ABS and stability control are off, so the wheels can lock in an emergency stop and the car will not help correct a skid. Drive gently and get it checked soon. If the red brake lamp or a low fluid warning is also on, Audi's manual says to check the fluid and not to drive on if it is low.
Why are the ABS and ESC lights on at the same time?
Both systems use the same wheel speed sensors and the same control unit. When one sensor signal is lost, the car disables both and lights both warnings.
Can low brake fluid turn on the ABS light?
Low fluid normally triggers the red brake warning, and it is often seen together with the ABS lamp. Check the reservoir first whenever brake warnings appear together.
Will clearing the code fix the ABS light?
Only if the fault was a one-time glitch. If a sensor, wire or bearing is failing, the code and the light come back, usually within a short drive.
Could new brake pads cause an ABS light?
The pads themselves do not, but the work can. A sensor knocked out of place or a cable pinched during a brake job is a common cause of a light that appears right afterward.
Sources
- Audi A5 Owner's Manual (ManualsLib) - ABS lamp monitors ABS and EDL; ABS with brake lamp indicates an ABS fault; pad wear symbol
- Audi owner's manual: Fault in the brake system - rear wheels can lock when ABS control is out of action; do not drive on with low fluid
- Audi A5 owner's manual: Electro-mechanical parking brake - parking brake fault warning and workshop advice
- HELLA Tech World: Check and change ABS and wheel speed sensors - sensor failure causes, symptoms and diagnosis steps
- Bosch Mobility: Wheel-speed sensor - how active Hall-effect sensors read an encoder and feed ABS and ESP
- NHTSA recall database, Audi A5 - recall campaigns listed for the A5 in this era
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