The ABS light and the traction control (or StabiliTrak/VSC) light usually come on together because both systems read the exact same wheel speed sensors through the exact same control module. When one sensor's signal goes bad or missing, the module can no longer run anti-lock braking safely, and it cannot run traction or stability control either, so it shuts both off and lights both warnings at once. The fix is almost always at the sensor, its wiring, or the ring it reads - not two separate problems.
Why One Fault Lights Two Different Warnings
Every wheel on a modern car has a wheel speed sensor. Bosch, which pioneered the electronic stability program still used across the industry, builds these sensors to "deliver information on the wheels' rotation characteristics to the ABS/ESP control unit." The anti-lock braking system uses that signal to release brake pressure the instant a wheel is about to lock up. The traction control and electronic stability system use the identical signal to tell when a wheel is spinning faster than the others and needs individual braking to regain grip. Both functions live in the same control module and depend on the same four sensor inputs, so a fault that corrupts or drops one wheel's signal takes down both functions together and both telltales come on at the same time. That shared-sensor design is exactly why a single wheel speed sensor problem, rather than two unrelated failures, is the most common explanation when both lights appear.
What Actually Fails: Two Real-World Examples
General Motors has documented this exact symptom combination in its own technical bulletins, and the two root causes it describes cover most of what a shop finds in the field.
The first is wiring damage. GM bulletin 16-NA-090 covers 2016-2017 Chevrolet Volts where customers reported the ABS warning light on, the Traction Control System/StabiliTrak light on, and Service Brake Assist and Service StabiliTrak messages on the driver information center, with diagnostic codes in the C0500 to C0516 range. GM's stated cause: "mis-routing or twisting of the front wheel speed sensor wiring harness, causing damage to the wires internally." The repair replaces only the sensor and harness found twisted or mis-routed, then reroutes the new harness to run above the brake line so it cannot twist again.
The second is contamination, not a dead part at all. GM bulletin PIC5428C, covering a wide range of 2007-2012 Buick, Cadillac, Chevrolet, GMC and Saturn models, describes the same combination of ABS, Service Traction Control and Service StabiliTrak lights with codes C0035 through C0050. On these vehicles the wheel speed sensor does not read a toothed metal tone wheel; it reads a magnetized nitrile rubber ring, built into the wheel bearing hub, made up of alternating north and south pole pairs. GM's finding: the lights were coming on because "ferrous metallic debris" - fine metal particles, usually from brake wear - had stuck to that magnetic ring and scrambled the signal. The bulletin is explicit that most of these cars needed nothing replaced: a technician identifies which corner's code is set, brushes the ring clean with a dry nylon brush (never a magnetic tool, which can damage the ring further), washes it with mild soap if needed, clears the codes and re-tests.
Those two cases bracket almost everything else that causes the combination: corrosion on the sensor tip or its electrical connector, a cracked or physically damaged sensor from a curb strike or a rock, a chewed wiring harness, or a worn wheel bearing that lets the sensor-to-ring air gap drift out of spec.
How a Shop Actually Finds the Cause
A scan tool is the starting point, not the finish line. The stored code points at a corner and a type of fault, but it does not prove the sensor itself is bad. Industry diagnostic guidance is blunt about this: wheel speed sensors are tested with a return rate around one in 200 units with nothing wrong, meaning shops that replace a sensor off the code alone are frequently replacing a good part.
The correct bench and vehicle test depends on which type of sensor is fitted. Older passive (variable-reluctance) sensors generate their own small AC signal and can be checked for resistance with a multimeter; newer active (Hall-effect) sensors run on a separate 12-volt supply and must never be resistance-checked, since that can destroy the sensor - they are checked for voltage instead. An oscilloscope settles the question outright: a healthy passive sensor shows a continuous sine wave that grows taller as wheel speed increases, while an active sensor shows a clean square wave switching between a fixed low and high current. Before any electrical test, a technician inspects the sensor tip and the ring or tone wheel it faces for contamination, corrosion, or physical damage, and confirms the mounting and air gap are within spec - exactly the debris check GM's own bulletin describes.
| What's found | Typical cause | Typical fix |
|---|---|---|
| Codes clear, light returns later | Debris on the tone wheel or magnetic encoder ring | Clean the ring; no parts needed in most cases |
| Harness visibly twisted or rubbing | Wiring damaged internally from mis-routing or chafing | Replace the sensor/harness, reroute away from moving parts |
| Sensor reads intermittently in cold or wet weather | Corrosion at the sensor tip or connector | Clean contacts or replace the sensor |
| No signal at all from one corner | Sensor physically cracked or wiring severed | Replace the wheel speed sensor |
Why You Shouldn't Just Live With It
Both lights on together means the car has quietly dropped back to a plain hydraulic brake system with no anti-lock protection and no stability or traction assist. Normal, dry-road braking is unaffected, but the exact situations these systems exist for - a hard stop, a wet or gravel surface, a sudden swerve - are the situations where the car will behave differently than the driver expects, with no system stepping in to help. It's not an emergency the way a brake warning light paired with low pedal or low fluid can be, but it shouldn't sit for months either, especially heading into wet or winter driving.
What Gets Replaced, and What Doesn't
If the diagnosis points to the sensor itself, replace just the sensor - most are a single bolt and a connector. If the fault is wiring, the repair is the harness section and, on vehicles like the Volt above, the sensor it plugs into, rerouted so it cannot twist against the strut or brake line again. If the cause is a worn wheel bearing letting the air gap drift, the hub and bearing assembly gets replaced, since on many of today's vehicles the encoder ring GM describes is molded into that same hub - the sensor was never actually the problem. What a shop should not do is replace a sensor on code alone without the contamination and wiring checks above; see how ABS works for how the sensor signal fits into the rest of the system, and shop wheel speed sensors and related brake hardware once the actual fault is confirmed.
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Shop Brake Parts →Frequently Asked Questions
Can I still drive with the ABS and traction control lights both on?
Yes, for normal driving - regular braking and steering still work. The risk is losing anti-lock and stability assistance in the exact moments they matter most, so get it diagnosed rather than ignoring it long-term.
Does one bad wheel speed sensor really affect both systems?
Yes. ABS and traction/stability control run from the same control module reading the same four wheel speed sensors, so a fault at any one corner typically disables and lights both warnings together.
Will clearing the code make the lights go off for good?
Only if the underlying cause is actually fixed. Clearing a code after cleaning debris off a sensor ring, as GM's own bulletin describes, can be a complete and permanent fix; clearing a code without finding the cause usually brings the light back.
Is it always the sensor, or could the module itself be at fault?
Usually not the module. The far more common causes are the sensor's wiring, corrosion, physical damage, or debris on the ring it reads - which is also why sensors tested after being returned under warranty are frequently found to have nothing wrong with them.
Why did the light come on right after new brakes or a wheel bearing?
A disturbed or mis-routed sensor wire, a sensor connector not fully seated, or a new hub's encoder ring not seating against the sensor at the correct gap are the usual causes after that kind of service.
Can I clean the sensor myself instead of replacing it?
On some vehicles, yes - GM's bulletin for a wide range of its own models calls for brushing debris off the sensor ring with a dry nylon brush and mild soap, never a magnetic tool. Whether that applies to a specific vehicle depends on its sensor design, so a shop should confirm it first.
Sources
- GM Technical Service Bulletin 16-NA-090, via NHTSA - 2016-2017 Chevrolet Volt ABS/TCS/StabiliTrak lights caused by twisted or mis-routed front wheel speed sensor wiring.
- GM Preliminary Information Bulletin PIC5428C, via NHTSA - ABS/traction/StabiliTrak lights across multiple GM models caused by ferrous debris on the magnetic wheel speed sensor encoder ring, and the clean-don't-replace repair.
- Bosch Auto Parts, Tire and Chassis - how wheel speed sensors feed a shared signal to both the ABS and ESP/traction control functions.
- Professional Motor Mechanic, How to Diagnose Wheel Speed Sensors - passive vs. active sensor test methods, warranty return-rate data, and common misdiagnosis.