2019-2024 Subaru Ascent ABS Light On: Common Causes and Fixes

By Core Brake Parts Tech Team — Circleville, OH warehouse

An ABS light on a 2019-2024 Subaru Ascent means the anti-lock system has found a fault and switched its ABS function off. Your normal brakes still work. The most common causes are a failed or dirty wheel speed sensor, damaged sensor wiring, or a problem with the tone ring. On early Ascents, a known software issue can also turn on the VDC, EyeSight and parking brake warning lights all at the same time. A code scan tells you which one you have.

How the ABS and VDC system works on the Ascent

The Ascent has a sensor at each wheel that reads a toothed or magnetic ring (the tone ring) as it spins. Those signals go to the ABS/VDC control module. If one wheel starts slowing faster than the others during a hard stop, the module pulses the brake pressure to that wheel so it keeps turning instead of locking up. Vehicle Dynamics Control (VDC), which is Subaru's stability control, uses the same sensors and the same hydraulic unit. Subaru's own warning-light guidance says the VDC system does its ABS control through the ABS system's electrical circuit. That is why one bad wheel speed signal often turns on more than one light.

On the Ascent, several other systems also depend on VDC being healthy: EyeSight driver assist, Reverse Automatic Braking and the auto-release of the electronic parking brake. If VDC shuts off, those features shut off with it, and each one turns on its own warning lamp. So when you see a whole row of lights on the dash, don't assume four different parts failed. It is often one fault spreading through connected systems.

What each warning light is telling you

Subaru dealer guidance says that with the ABS light on, the ABS function shuts down and the conventional brakes keep working normally. It also says to have the system checked if the ABS light stays on while driving or doesn't come on briefly at startup. The red brake system warning light is more serious. Besides the parking brake, it warns about low brake fluid and electronic brake force distribution faults. Subaru's guidance says driving with that light on is dangerous.

Common causes of an Ascent ABS light

1. Wheel speed sensor or its wiring

This is the most common cause on most vehicles, and the Ascent is no different. The sensors sit behind each wheel in the path of water, road salt and brake dust. Delphi lists the usual ways they fail: dirt building up on the sensor or tone ring, wires breaking from vibration, internal shorts, and wheel bearings that have worn loose. The sensor harness runs along the suspension and flexes every time the wheel moves. A wire can break inside its insulation and cause a fault that comes and goes.

2. Metal debris on the sensor tip

The sensors are magnetic, so they pick up metal particles from worn brake parts. A coating of brake dust and rust can weaken the signal enough to set a code. Cleaning the sensor tip and the tone ring sometimes fixes the problem without any new parts.

3. Tone ring or wheel bearing problems

If teeth on the tone ring are chipped or cracked, the sensor gets a weaker pulse every time those teeth pass. That looks like a wheel that is speeding up and slowing down. A worn wheel bearing lets the hub move, which changes the gap between the sensor and the ring and makes the signal erratic. If a wheel speed code keeps coming back after you replace the sensor, check the hub and bearing on that corner.

4. Damage during a brake or suspension job

Delphi specifically names damage done while replacing other brake parts as a cause. On the Ascent, it's easy to stretch or pinch a sensor lead while swinging the caliper out of the way or pulling a hub. If the light came on right after a pad, rotor or hub job, check that corner first.

5. Low brake fluid

Pads get thinner as they wear, and the caliper pistons move out to follow them. That pulls the fluid level in the reservoir down. Low fluid turns on the red brake light and can show up together with ABS warnings. If the level is low and there is no leak, the pads are probably worn. Top it up with the fluid type printed on the reservoir cap and in your owner's manual, then inspect the pads.

6. Mismatched tire sizes

The ABS module compares the speed of all four wheels. A spare or a replacement tire with a different rolling diameter makes one wheel read fast all the time. This matters even more on an all-wheel-drive Subaru, where matched tires are important for the drivetrain too.

The 2019-2020 Ascent VDC software issue

Subaru issued Service Program bulletin WUU-06R (revised December 2019) for certain 2019-2020 Ascents, along with some 2020 Legacy and Outback models. The bulletin says the original engine computer software could repeatedly learn the accelerator position incorrectly, which could disable the VDC function. With VDC disabled, EyeSight, Reverse Automatic Braking and the electronic parking brake auto-release are disabled too, and all four warning lamps come on. The bulletin's subject is ECM reprogramming for DTC C1424. The fix is a software update at a Subaru retailer, not a part replacement.

If you have a 2019 or 2020 Ascent with that group of lights on, and especially if the only code is C1424, ask a Subaru dealer to check coverage by VIN before you buy sensors. Subaru says not every vehicle in the production range is covered, so the dealer has to confirm coverage.

How to diagnose it step by step

1. Check the brake fluid level with the vehicle parked on level ground. If it's low, find out why before you do anything else.
2. Read the codes. A basic engine-code reader often can't see ABS codes. You need a scan tool that reads ABS/VDC modules. Write the codes down before you clear anything.
3. If a wheel speed code points to one corner, look at that sensor and its harness for rubbing, cuts, corrosion at the connector and packed-on debris. Flex the wiring while watching live data. An intermittent break often shows up that way.
4. Compare live wheel speeds during a slow test drive. A sensor that drops to zero or jumps around while the others read steady is the problem.
5. Inspect the tone ring and bearing on that corner if a new sensor doesn't fix it.
6. For a dash full of lights with a C1424 code, ask the dealer about the software service program first.

The fix, what to replace, and how hard it is

Replacing a wheel speed sensor is usually a moderate DIY job. You unbolt the sensor, unclip the harness from its brackets, and reconnect the new one routed exactly like the old one. The most common mistake is not following the original clip path. A loose lead can rub on a tire or suspension part and fail again. Clean the sensor bore before installing the new sensor. Don't force a seized sensor out with a screwdriver, because the tip can break off inside the knuckle.

If the tone ring is part of the hub or bearing, a bad ring means replacing the hub assembly. That is a bigger job and usually calls for correct torque values from the factory service manual. Tighten every fastener to the value in the Subaru service information. Don't guess.

If the light came on after the brakes were worn down to metal, fix the brakes too. Metal-on-metal wear throws debris onto the sensors and pulls the fluid level down. Replace pads and rotors as a set on the axle. We carry front ceramic brake pads that fit the 2019-2024 Subaru Ascent, along with replacement brake rotors and brake hardware kits so the new pads slide freely. The rear brakes use the electronic parking brake. Put the EPB into service mode with a scan tool or the manual's procedure before you push the rear pistons back, or you can damage the actuator and set more warning lights.

After any repair, clear the codes and take a test drive long enough for the module to see every wheel turning. The ABS light should come on briefly at startup and then go out. That short flash at key-on is the system testing itself and is normal.

Frequently Asked Questions

Can I drive my Ascent with the ABS light on?

If only the ABS light is on, your normal brakes still work, but the wheels can lock in a hard stop. Drive gently, leave more following distance and get it scanned soon. If the red BRAKE light is also on, don't drive it until the cause is found.

Why did the ABS, VDC and EyeSight lights all come on at once?

EyeSight, Reverse Automatic Braking and the electronic parking brake auto-release all depend on VDC. When VDC shuts off, those systems shut off with it and each turns on its own lamp. One wheel speed sensor fault, or the ECM software issue covered by bulletin WUU-06R on some 2019-2020 Ascents, can cause all of them.

Will disconnecting the battery clear the ABS light?

It might turn the light off until the next drive, but the module sets the fault again as soon as it sees the problem. The fault codes are the clue that leads to the fix, so read and record them before you clear anything.

Can new brake pads turn off the ABS light?

Not directly. Pads don't have ABS sensors. But badly worn pads can lower the fluid level and throw metal debris onto the wheel speed sensors, and both of those can trigger warnings. Fixing the brakes and cleaning the sensors can clear those faults.

Do I need a special scan tool for the Ascent's ABS?

You need a tool that can read the ABS/VDC module, not just engine codes. You'll also want one that can put the electronic parking brake into service mode if you plan to work on the rear brakes.

Sources

1. Subaru Service Program Bulletin WUU-06R (NHTSA) - 2019-2020 Ascent ECM software can disable VDC, EyeSight, RAB and EPB auto release; DTC C1424; dealer reprogramming
2. Subaru warning lights guide (Harvey Subaru) - ABS shuts down while conventional brakes work; brake light covers parking brake, low fluid and EBD; VDC uses the ABS circuit
3. Delphi Technologies: Making sense of your sensors - ABS sensor - how wheel speed sensors work, symptoms, failure causes and diagnosis
4. Brake and Front End: Diagnosing wheel speed sensors - active vs passive sensors, tone ring damage, wiring faults, brake debris and tire size effects