Test a wheel speed sensor (ABS sensor) by measuring its resistance with a multimeter: with the sensor disconnected, touch the leads to its terminals. A passive (2-wire) sensor should read 800 to 2000 ohms. An open circuit, a short, or a constantly changing reading means the sensor is faulty.
The ABS light usually comes on when a sensor fails, because a fault in any one part makes the whole anti-lock braking system stand down. Since the sensor is a common point of failure, testing it first is a smart move.
Step 1: Inspect the Sensor
Put the vehicle on ramps and check the sensor wiring underneath — in many cases you can do this without removing a wheel. Frayed, corroded, or snapped wires are a direct cause.
Step 2: Clean the Sensor
Road grit and iron filings can cling to the sensor and block its signal. Taking the sensor off and wiping it clean usually fixes this, with no chemical cleaner needed. If cleaning does not help, move on to the multimeter test.
Step 3: Test the Sensor With a Multimeter
A sensor can pass a visual check yet still be dead internally. With the sensor unplugged, touch the multimeter leads to its terminals — this tests it without taking the sensor off the car. Both passive (2-wire) and active (2 or 3-wire) sensors can be checked this way; look up which type your vehicle uses first.
On a passive sensor, a steady reading of 800 to 2000 ohms across the signal and ground leads indicates a good sensor. A reading that constantly changes or climbs, an open circuit (no resistance), or a short all point to a faulty sensor, often from an internal short, a cracked housing, or moisture.
For access to the wiring without removing a wheel, check near the inner strut towers for front sensors, and under the rear seat cushion or in the trunk for rear sensors.
What to Do After Testing
If the test confirms the sensor is faulty, replace it. A faulty sensor disables the ABS and also affects the traction control and stability control systems that rely on its data. Purchase a replacement before removing the faulty sensor so the vehicle stays drivable.
