AI Mechanic

Symptom · Any vehicle with four wheel speed sensors

ABS and traction light on together

ABS and traction share the same wheel speed signals, so one bad sensor lights both. The whole job is deciding which corner is wrong and whether the sensor, the ring or the wiring is to blame. Live data answers that from the driver's seat.

01Possible causes, none of them ranked yet

  • Failed wheel speed sensor

    An open or shorted sensor reads zero while the wheel is clearly turning.

  • Damaged or contaminated magnetic ring

    A rusted or scored ring inside the bearing gives a signal that drops in and out with speed.

  • Chafed sensor wiring at the suspension

    Steering and suspension movement breaks the loom where it flexes, so the fault comes and goes.

  • Corroded sensor connector

    Water in a wheel arch plug raises resistance and the signal fails intermittently.

  • Worn wheel bearing

    Excess play changes the air gap, so the signal weakens as the wheel loads in a turn.

  • Low battery voltage or a bad ground

    The ABS module drops out and sets codes on several wheels at once with no sensor fault at all.

Nothing on that list is the answer yet. A measurement decides which ones survive.

02The tests, in the order a senior tech runs them

Test 1

Read stored ABS codes and note which wheels they name, then clear and drive to see which returns.

tool
Scan tool with ABS access
expect
One wheel named repeatedly, not all four.

One wheel means a corner fault. Codes on three or four wheels point at supply, ground or the module.

Test 2

Graph all four wheel speeds on a slow drive, watching for one that drops to zero or spikes.

tool
Scan tool with graphing live data
expect
Four traces tracking within a few km/h of each other from walking pace upward.

A trace that reads zero below about 10 km/h and then joins in is a classic weak signal or a bad air gap, not a dead sensor.

Test 3

Wiggle the loom at the strut and turn the steering lock to lock while watching that wheel's live value.

tool
Scan tool and a helper
expect
The reading stays steady through the full range of movement.

A value that cuts out on movement puts the fault in the wiring and saves you a sensor and a hub.

Test 4

Unplug the suspect sensor and measure its resistance, then compare with the same sensor on the other side.

tool
Multimeter
expect
Passive sensors: roughly 800 to 2500 Ω, within about 20 % of the opposite side. Active sensors: check supply voltage instead, usually 8 to 12 V.

An open circuit or a wildly different reading from its pair condemns the sensor on evidence.

Test 5

Pull the sensor, inspect the tip and the magnetic ring, and check the hub for play with the wheel off the ground.

tool
Torch, mirror, and hands at 12 and 6 o'clock
expect
A clean tip, an unbroken ring, no perceptible play at the hub.

Swarf on the tip or a rusted ring means the sensor was reading a damaged target, and a new sensor alone will not fix it.

Codes name the corner, live data proves it, and a wiggle test decides between wire and sensor. Only then does anything come off the car.

Your car is not this car.

Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.

Open your first Case

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