ABS and Traction Control Lights On Together: What It Means and What Fails

Your anti-lock brakes, traction control and stability control are not three separate systems. They are three functions of one controller, the ABS module, working from one set of inputs, the wheel-speed sensors at each corner. Traction control cuts engine power or applies a brake when one wheel spins faster than the others. Stability control compares what the wheels are doing against the steering-angle and yaw sensors and brakes individual wheels to keep the car pointed where you are steering.

All of that depends on trusting the wheel-speed data. The moment the module sees a fault, whether a sensor dropping out or a problem inside the module itself, it shuts down anti-lock function and everything built on top of it. Two or three lights appear at once because one input failed, not because three things broke.

On many cars a brake warning light joins them, and on some you lose cruise control and hill-hold as well, since those features also lean on wheel-speed data.

In rough order of how often they turn out to be the cause:

• Wheel-speed sensor. A small sensor at each hub that reads a toothed ring and reports how fast that wheel is turning. They fail from heat, internal corrosion or physical damage, and one bad sensor is the single most common reason for this pair of lights.
• Tone ring. The toothed ring the sensor reads. On some designs it is pressed onto the axle or CV joint and can rust, crack or shed teeth, which corrupts the signal.
• Sensor wiring. The harness to each sensor flexes with the suspension and lives in salt spray. Chafed insulation, a corroded connector or a wire broken inside its jacket produces a fault that is often intermittent at first.
• Hub bearing with a built-in sensor. Many Ford, GM, Honda, Toyota, Nissan and Subaru models put the sensor and ring inside the wheel bearing. When the bearing wears, the signal goes bad with it, so the repair is a bearing rather than a sensor.
• Brake fluid level or a brake-light switch. On some cars a fault on the brake-system side disables ABS and traction control as well, so the lights do not always start at a wheel.
• Steering-angle sensor. Stability control needs to know where the wheel is centered. A sensor that has lost its reference or failed puts the stability light on even when the wheel speeds are fine.
• The ABS module itself. Internal solenoid or pump-motor faults, a failing processor or corroded pins at the module connector. Less common than a sensor, and the most expensive, which is why it should be confirmed rather than assumed.

Massachusetts roads are hard on exactly these parts. A pothole hit at speed can bend a sensor bracket, crack a tone ring or shock a wheel bearing enough to start it on its way out. The lights may not come on that day, but the damage shows up over the following weeks as the bearing loosens or the ring corrodes at the crack.

Road salt does the slower work. Brine wicks into sensor connectors and along the wire strands, and the corrosion raises resistance until the signal becomes noisy or disappears. Rust also builds up between a sensor and its mounting hole, pushing it out of position, and it eats the steel tone rings on the axle shaft.

The result is a lot of ABS faults that appear in late winter and early spring, often intermittent at first. Note the conditions when your lights come on, whether after a bump, in wet weather, at startup or only above a certain speed. That pattern does not tell you the part, but it does tell the technician where to start, and it is the one thing you can contribute to the diagnosis from the driver's seat.

Most inexpensive code readers talk only to the engine computer. ABS and stability codes live in a different module, use manufacturer-specific code sets, and are simply not visible to a basic reader. The scan comes back clean while the lights stay on.

Even a reader that can see a chassis code gives you a general description like wheel speed sensor circuit. It cannot tell you whether that circuit fault is a dead sensor, an open wire, a corroded connector or a bearing that has worn. Those are four different repairs at four different prices, and a description that fits all four is not a diagnosis.

The same goes for the parts counter. A sensor bought on the strength of a code description fixes the car only when the sensor happened to be the fault. When it was the wiring or the bearing, the lights come straight back with the new part installed, and the cost of the guess is added to the cost of the real repair.

A factory-level scan tool running the manufacturer's own diagnostic software talks to the ABS module directly, on every make we see. That changes what we can see.

The module's own stored codes name the corner and the type of fault, along with the conditions recorded when it set. From there we check the sensors, the wiring and the module on the car and on a road test, following the manufacturer's own test methods. That separates a sensor fault from a wiring fault and a bearing problem before any part is ordered. If the wheel-speed side is clean, the same tool shows whether the steering-angle and stability sensors are reporting properly or need attention. And before anyone suggests replacing the module, the module itself is confirmed healthy or faulty rather than assumed.

You leave with a written report that names the corner, the part and the repair. The whole process starts at $150 for the Full System Diagnostic and is usually finished the same day.

With an ABS fault, the hydraulic brakes themselves almost always work normally. Press the pedal and the car stops. What you lose is the anti-lock function, so in a hard stop on a wet or icy road the wheels can lock and slide instead of pulsing. You also lose traction and stability control, which matters most on snow, in rain and in sudden lane changes.

Drive as if the car is from before those systems existed. Leave more following distance, brake earlier and more gently, and take corners slower in bad weather. If the red brake warning light is also on, or the pedal feels soft or sinks, stop driving and have the car checked right away, because that points to a hydraulic problem rather than an electronic one.

Do not let it ride for months. An intermittent sensor fault tends to become a permanent one, and a worn bearing that is upsetting a sensor today can fail outright later.

Note when the lights come on: at startup, after a bump, in wet weather or only above a certain speed. Then bring the car to our shop for a Full System Diagnostic in Massachusetts from $150. We read the ABS module directly, confirm the fault on the car and give you a written report with the exact corner and part.

Drivers north and west of the city can start from our Waltham diagnostics page or Lowell diagnostics page. Call (617) 383-9331 or request a free quote and we reply within 2 hours with a firm price.