By Core Brake Parts Tech Team — Circleville, OH warehouse
If the ABS light stays on in your 2017 Subaru WRX STI, the anti-lock function has switched itself off, but your normal brakes still work. The most common causes are a failed or contaminated wheel speed sensor, damaged sensor wiring, worn wheel bearings, or low battery voltage. If the red BRAKE light comes on at the same time, treat it as more serious: the electronic brake force distribution system may be down, and you should check the brake fluid before you drive any further.
How the ABS system works on the 2017 WRX STI
Subaru lists a Brembo Performance Brake System with Super Sport ABS as standard on the 2017 WRX STI, working alongside Multi-Mode Vehicle Dynamics Control, Active Torque Vectoring and the multi-mode Driver Controlled Center Differential (DCCD). All of those systems depend on one thing: accurate wheel speed readings from a sensor at each corner of the car.
Each wheel speed sensor reads a toothed encoder ring (tone ring) that turns with the hub. The ABS control unit compares all four signals many times a second. When one wheel slows faster than the others under braking, the hydraulic unit releases and reapplies pressure to that wheel so it keeps turning. If the control unit sees a signal that is missing, erratic or implausible, it cannot trust its data, so it shuts ABS off, stores a fault code and turns on the amber ABS warning light.
Because the STI's stability control and torque vectoring use the same wheel speed data, one bad sensor can turn on more than one lamp. Seeing the ABS light together with the VDC warning, or a DCCD indicator acting strangely, usually points to the same root cause rather than several separate failures.
What the warning lights are telling you
The owner's manual for this generation of WRX STI explains that the ABS light should come on when you switch the ignition on and go out about 2 seconds after the engine starts. Anything else needs attention. The table below sums up the light combinations you may see.
| What you see | What it means | What to do |
|---|---|---|
| ABS light on, BRAKE light off | ABS is shut down; conventional braking still works normally | Drive gently, avoid hard stops, and get the fault code read |
| ABS light and BRAKE light on together | Possible fault in the EBD (electronic brake force distribution) system; rear wheels are more prone to locking under hard braking | Stop safely and follow the check procedure below |
| BRAKE light only, parking brake released | Brake fluid level has dropped to near the MIN mark | Check fluid level and look for leaks or badly worn pads |
| ABS light on after a jump start | Low battery voltage, not a fault | Light should go out once the battery is fully charged |
| ABS light never comes on at key-on | Bulb or system self-check problem | Have the system inspected |
When ABS and BRAKE come on together while you are driving, Subaru's procedure is: stop on flat, safe ground, shut the engine off, set the parking brake and restart. If both lights go out after you release the parking brake, drive carefully to a shop. If they come back, stop again and check the brake fluid. If the fluid is at or above MIN, have the system inspected. If it is below MIN, do not drive the car; have it towed.
The most common causes
1. Wheel speed sensor faults
This is the most frequent cause on any AWD performance car. HELLA's technical guidance lists the typical sensor failures as broken wiring, internal short circuits, external damage, heavy contamination, increased wheel bearing play and mechanical damage to the encoder ring. On an STI, the sensors live close to large, hot Brembo calipers and take a beating from road salt, water and brake dust, so corroded connectors and chafed harnesses are common, especially on cars that see winter roads or track days.
2. Damaged wiring or connectors after brake or suspension work
Sensor leads are routed along the strut and knuckle. A harness that was pulled, pinched or left unclipped during a pad, rotor, coilover or hub job can break internally and produce an intermittent fault that only shows up over bumps or at speed. If the light appeared right after work on one corner, start your inspection there.
3. Worn wheel bearings or a damaged tone ring
The sensor has to sit a precise distance from the encoder ring. A wheel bearing with excess play lets that gap change as the wheel turns, and the signal becomes erratic. A cracked, rusty or packed-with-debris tone ring does the same thing. If you also hear a hum that changes with speed, or feel play when you rock the wheel, check the bearing before buying a sensor.
4. Low battery voltage
The owner's manual notes the ABS light may come on after a jump start because of low battery voltage, and that it should go out once the battery is charged. A weak battery, a loose ground or a failing charging system can trigger ABS and other warning lights with nothing wrong in the brakes themselves.
5. Low brake fluid or a hydraulic problem
Low fluid primarily turns on the red BRAKE light, but when it appears together with the ABS light, it signals that the EBD function may be affected. Fluid level falls slowly as pads wear, so a low reservoir on a car with thin pads is expected; a sudden drop means a leak at a caliper, line or hose that must be fixed before the car is driven.
6. ABS hydraulic unit or control module
The module and hydraulic unit can fail, but they are the least likely cause and the most expensive part. Rule out every sensor, wire, connector and bearing first.
How to diagnose it step by step
- Read the codes. ABS faults are stored in the brake control unit, not always the engine computer, so you need a scan tool that can read chassis or ABS codes. The code will usually name the corner (front left, rear right and so on) and whether the problem is the circuit or the signal.
- Check the battery and charging system. A weak battery can set false codes. Charge it, clear the codes and see whether the light returns.
- Check the brake fluid level and look for wet spots at each caliper and hose.
- Inspect the flagged corner. Follow the sensor harness from the sensor to the body connector and look for chafing, broken clips and green corrosion in the plug. Pull the sensor and look for metal debris or rust on the tip.
- Test the sensor. HELLA recommends checking supply voltage and signal with a multimeter and, ideally, an oscilloscope. A scan tool with live data is the easiest way for most owners: drive slowly and compare all four wheel speeds. A wheel that reads zero or jumps around is your problem.
- Check the bearing and tone ring for play, damage or buildup if the sensor and wiring test good.
The fix and what to replace at the same time
Most STI ABS lights are cured with a new wheel speed sensor or a wiring repair. When installing a sensor, HELLA advises cleaning the sensor mounting point thoroughly first, since rust or debris under the sensor changes the gap and can bring the fault right back. Clear the codes afterward and confirm on a test drive that the light goes out once the car is moving.
If the corner needs a hub bearing, that is a good time to inspect the brakes on that axle, because the caliper and rotor come off anyway. Check pad thickness and rotor condition while you are in there. If the pads are near their limit, the hub is apart and the rotors are worn or grooved, replacing brake pads and brake rotors on that axle saves a second teardown. STI owners who track their cars often move to a performance rotor such as the DBA 4000 Series XS front rotor for the 2017 and newer STI. Always replace pads and rotors in axle pairs, never on one side only.
If you had a fluid leak, fix the leaking part, then bleed the system with the fluid type printed on the reservoir cap and in your owner's manual. A leaking flexible hose is a good reason to replace both hoses on that axle; see our brake hoses and brake parts collections.
How hard is the job?
Reading codes and checking the battery and fluid is easy for any owner. Replacing a wheel speed sensor is usually a moderate job: the wheel comes off, the sensor is held by a bolt, and the harness is clipped along the strut. Seized sensors are common on older cars in salt country and may need penetrating oil and patience. Hub bearing replacement is harder and may need a press or a bolt-in hub assembly depending on the corner. Any work on the ABS hydraulic unit or brake lines requires correct bleeding, and on some ABS systems that requires a scan tool to cycle the valves, so leave it to a shop if you are not equipped. Torque every fastener to the value in the factory service manual.
For recalls: the NHTSA recall database lists one campaign for the 2017 WRX line (22V522000, a corroding backup light switch), and it is not brake related. An ABS light on this car is a maintenance and repair issue rather than a known recall defect, but it is still worth running your VIN through NHTSA before you start.
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Shop Brake Parts →Frequently Asked Questions
Is it safe to drive a 2017 STI with the ABS light on?
If only the ABS light is on and the BRAKE light is off, the owner's manual says conventional braking works normally, but you have lost anti-lock control. Drive gently, leave extra following distance and get it repaired soon. If the BRAKE light is also on, stop and follow the fluid check procedure before driving further.
Why are the ABS and VDC lights on at the same time?
Stability control and traction control use the same wheel speed signals as ABS. When one sensor fails, the car disables all the systems that rely on it, so several lights come on for one fault.
Will disconnecting the battery clear the ABS light?
It may turn the light off briefly, but if the fault is still there the control unit will find it again on the next drive and relight it. The only exception is a light caused purely by low voltage, which goes out once the battery is charged.
Can new brake pads or rotors cause an ABS light?
The pads and rotors themselves do not, but the job can. A sensor harness pulled or pinched during the work, or a sensor knocked out of position, will set a code. Check the wiring on the corner you worked on first.
Do I need to replace the ABS sensor on both sides?
No. Unlike pads and rotors, wheel speed sensors are replaced individually. Replace the one the code and live data point to.
Sources
- Subaru WRX STI Owner's Manual - ABS warning light - light behavior at start-up; ABS shuts down while conventional brakes still work
- Subaru WRX STI Owner's Manual - Brake system warning light - fluid level near MIN, parking brake indicator, low battery voltage after a jump start
- Subaru WRX STI Owner's Manual - EBD warning - ABS and BRAKE lights together, rear wheel lock risk, stop-and-check procedure
- Subaru U.S. Media Center - 2017 WRX and WRX STI announcement - Brembo brakes with Super Sport ABS, Multi-Mode VDC, Active Torque Vectoring, DCCD
- NHTSA recall database - 2017 Subaru WRX - only listed recall is 22V522000, backup light switch
- HELLA Tech World - Check and change ABS and wheel speed sensors - sensor failure causes, testing and installation tips
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