the sensors are fitted into a slot on the wheel hub accessible from engine side of the wheel hub. easiest to remove wheel and look for insulated wire coming down through the wheel well. there is 5mm? hex bolt holding it in place.

if there is a lot of crud in the slot you may have a hell of time getting the sensor out to clean.

the tone ring is FRAGILE and part of the wheel hub. it is slotted metal wheel which spins at VERY CLOSE proximity to the magnetic end of the sensor creating a magnetic field and electrical signal.

IF the sensor does not easily come out of its slot you will have to do this to clean and check it.

1. remove wheel
2. remove brake pads, unbolt caliper and hang from a wire on the coil spring out of the way
3. remove rotor exposing the tone ring and sensor end

now you can see what you are doing to carefully and brush both.

this may or may not help you to decide which sensor is offending.

The basic diagnostic for the front sensor wiring is as follows:

-Open a front sensor connector in the wheel well. The resistance between the wires going TO the sender IN the wheel should be .9-2.2 KOhms.