2006-2018 BMW 335i ABS Light On: Common Causes and Fixes

By Core Brake Parts Tech Team — Circleville, OH warehouse

When the ABS light comes on in a BMW 335i, the usual culprit is a failed or disturbed wheel-speed sensor, and it rarely comes on alone: because the same sensors feed Dynamic Stability Control (DSC), you will typically see the ABS, DSC and often the yellow brake lamp together. Low brake fluid, damaged sensor wiring, a damaged encoder ring or wheel bearing, and faults inside the DSC module itself round out the list. A BMW-capable scan tool that reads the DSC module will point you to the right corner in minutes.

How the ABS and DSC System Works on the 335i

On the 335i, ABS is not a standalone system. It is one function of the DSC control unit, which combines the hydraulic valve block, pump and control electronics in one module. BMW's own technical training material for the E90-generation 3 Series (the 2006-2012 E90, E92 and E93 sedan, coupe and convertible) describes the US-market unit as the MK60E5, built by Continental Teves. The "E5" refers to five integrated brake pressure sensors: one on the inlet side that checks the master cylinder pressure, and one for each wheel brake.

Feeding that module are four active wheel-speed sensors, one per wheel, which BMW notes can also detect the direction the wheel is turning. Alongside them, DSC reads a steering angle sensor in the steering column switch cluster, a yaw rate and lateral acceleration sensor, the brake light switch and a brake fluid level switch in the reservoir.

The wheel-speed signal does more than run ABS. BMW uses the same four signals for DSC stability control, traction control, the Run Flat Indicator (the tire pressure warning compares each wheel's speed to the average) and brake-disc functions such as Fading Support and Brake Disk Drying. That is why one bad sensor can light up several warnings at once. The 2013-2018 F30-generation 335i uses a newer version of DSC, but the basic principle of one wheel-speed sensor per wheel feeding the stability control module is the same, so the diagnosis below applies to both generations.

Common Causes of the ABS Light on a 335i

Wheel-speed sensors

This is the most common cause by a wide margin. The sensors live right behind the brake rotor at each hub, where they see heat, water, road salt and brake dust every day. According to AA1Car's diagnostic guide, the most common sensor problems are a damaged tip or an internal failure, followed by corroded connectors and damaged wiring. When the DSC module stops getting a believable signal from one wheel, it sets a code for that wheel and shuts down ABS and stability control as a precaution.

Brake jobs are a frequent trigger. A sensor that is pried out carelessly, a harness that gets stretched when a caliper hangs from it, or a connector knocked loose while swapping rotors can all set the light the first time the car is driven afterward.

Encoder ring and wheel bearing

The sensor reads a magnetic encoder that turns with the hub. AA1Car points out that a worn wheel bearing changes the gap between the sensor and the encoder and makes the signal fluctuate, and that even small, hard-to-see damage to the encoder can upset the reading. If a new sensor does not fix the fault at one corner, check that wheel bearing for play or noise.

Low brake fluid

BMW's training material states that the reservoir has a reed-type level switch and that DSC is deactivated when brake fluid is too low. Low fluid is usually a symptom, not a cause: as pads wear, the caliper pistons move out and the level in the reservoir drops. If the level is low, check pad thickness and look for a leak before simply topping it off.

The DSC module

When all four wheel-speed signals read correctly in live data and the module still stores internal faults (pressure sensor, valve or pump codes), the problem is in the DSC unit itself. This is the least common and most expensive cause, so rule out the sensors, wiring and fluid level first.

How to Diagnose It

1. Read the codes from the DSC module. A generic engine-code reader often cannot see BMW's DSC module. You need a scan tool that supports BMW chassis modules. The fault will usually name the specific wheel.
2. Watch the live wheel speeds. AA1Car recommends comparing all four wheel-speed readings while driving straight. A wheel that reads zero, drops out, or disagrees with the other three is your suspect.
3. Do not rely on an ohmmeter. The 335i uses active sensors that need power from the module. AA1Car notes that resistance testing does not work on active sensors the way it does on older passive (magnetic) sensors; you have to verify the supply and the signal with the wheel turning.
4. Inspect the corner. Look at the sensor tip for metallic debris, check the connector for corrosion, and follow the wire for chafing or a broken clip. Bosch advises keeping wheel-speed sensors clean of debris, ice, snow and dirt.
5. Check the bearing and brake fluid. Rock the wheel for play, listen for bearing noise, and check the reservoir level.

The Fix and Which Parts to Replace

For a single bad sensor, replacement is simple: the sensor is held in the knuckle or hub carrier by a retaining bolt and plugs into a connector nearby. Clean the bore before installing the new one so it seats fully, route the wire exactly as the original was clipped, and tighten the bolt to the value in the factory service manual. Clear the codes and drive the car; the DSC module needs to see the wheel turning before the lights go out.

If the fault followed a brake job, go back and check that corner first. And if you are about to do brakes on a 335i, this is the time to take care with the sensors. The front and rear pads on this car also carry BMW's pad wear sensors, which feed the Condition Based Service display; BMW's training material says the DSC module uses reference points at 6 mm and 4 mm on those sensors to estimate remaining pad life. A pad wear sensor that has been cut through by a worn pad should be replaced with the new pads.

Parts we carry for this car:

• EBC Bluestuff NDX front brake pads for the 2006-2012 BMW 335i (E90/E92/E93)
• EBC Bluestuff NDX rear brake pads for the 2006-2012 BMW 335i (E90/E92/E93)
• EBC Bluestuff NDX rear brake pad set for the 2013-2018 BMW 335i (F30)

For other brake parts for the 335i, browse our brake pads, brake rotors and Bosch parts collections.

What to Replace at the Same Time

• Wheel bearing: if the encoder or bearing is the root cause, a new sensor alone will not hold.
• Brake fluid: if the level was low because of worn pads, replace the pads. If it was low because of a leak, fix the leak. Change the fluid on the interval in BMW's maintenance schedule.
• Pad wear sensors: replace any wear sensor that has been worn through or damaged whenever you replace pads.
• Clips and grommets: a sensor wire that rubbed through once will do it again if the retaining clip is missing.

How Hard Is the Job?

Replacing a wheel-speed sensor is one of the easier jobs on the car: wheel off, a retaining bolt, a connector, and routing the wire. The rear sensors are usually more work than the fronts because of corroded bolts and tighter wire routing along the rear suspension. The hard part is diagnosis without the right scan tool; guessing and replacing all four sensors is a waste of money when the DSC module will tell you exactly which wheel it is unhappy with.

Bearing replacement and DSC module repair are bigger jobs. A DSC module replacement typically requires the brake system to be bled with a scan tool that can cycle the module's valves, and the steering angle sensor may need to be initialized afterward, which BMW's training material lists as a diagnostic-equipment procedure.

Driving with the ABS light on: your normal brakes still work, but ABS and stability control are off. The car will lock wheels in a hard stop and will not intervene in a skid, which matters on a powerful turbocharged car in rain or snow. If the red brake warning is also on, check the fluid level before driving at all.

Frequently Asked Questions

Why are the ABS, DSC and brake lights all on at once on my 335i?

Because ABS is a function of the DSC module and all of them depend on the same four wheel-speed sensors. When one sensor signal is lost, the module switches off ABS and stability control together and lights the related lamps. One faulty sensor is the most common reason for the three-light combination.

Can a brake job cause the ABS light to come on?

Yes. A wheel-speed sensor that was pulled out, a connector that was bumped loose, or a wire that was stretched while the caliper was off can all set a fault. If the light appeared right after brake work, inspect the corners that were worked on first.

Will a cheap OBD2 reader find the ABS code?

Usually not. Basic code readers see engine and emissions codes. The ABS and DSC codes live in BMW's DSC module, so you need a scan tool that supports BMW chassis modules.

Can I test the 335i wheel-speed sensor with a multimeter?

Not reliably. These are active sensors that need a supply voltage from the module, so a simple resistance check does not tell you whether they work. Live wheel-speed data on a scan tool, or checking supply and signal with the wheel turning, is the right test.

Is it safe to drive with the ABS light on?

The base hydraulic brakes still work, but ABS and stability control are disabled, so the wheels can lock in a panic stop. Drive gently and get it diagnosed. If the red brake warning is on too, check the brake fluid before driving.

Can low brake fluid turn on the DSC light?

Yes. BMW's own training material says DSC is deactivated if there is insufficient brake fluid. Low fluid often means worn pads or a leak, so find out why it is low.

Sources

1. BMW Technical Training - E90 Dynamic Driving Systems - MK60E5 DSC module by Continental Teves, five pressure sensors, four active wheel-speed sensors, DSC deactivated with low brake fluid, Run Flat Indicator, pad wear sensor reference points, steering angle sensor initialization
2. Bosch Mobility - Wheel-speed sensor - how active wheel-speed sensors work and that ABS and stability control depend on their signal
3. AA1Car - Diagnosing ABS Wheel Speed Sensors - common sensor failure causes, encoder and bearing effects, live-data comparison, active sensors not testable by resistance alone
4. Bosch Auto Parts - Wheel Speed Sensors - keep sensors clean of debris, ice, snow and dirt