When the brake light comes on, most people go to the mechanic. Nobody wants the brakes to fail. If you have ABS, the fault is often the ABS sensor. This sensor tells the braking system how fast the wheels are spinning--and the braking system uses this information to come between you and the brakes when you are doing something that is going to lock them up. The sensor is easy to find and easy to check, and you can buy one at the auto supply store for a few bucks. With a little effort you might save hundreds of dollars.
Look in your car manual to see what the resistance is on tour Anti-locking Breaking System (ABS) sensor. It is 50 ohms on most cars. If you have ABS on the front wheels only you will be checking the sensors for the front wheels only. If you have ABS on all four wheels you will be doing this for all four wheels. Consulting the manual will tell you which arrangement you have. Unfortunately, you will have to replace the wheel and lower it each time. The brake light does not indicate which sensor is causing the problem. Do not stop checking if you find a bad one--you might as well check them all.
Jack up the wheel you are checking and take off the wheel. The ABS sensor is a little canister with a wire coming out of it. Disconnect the wire and unscrew the canister. You can clean it off with a rag with a little brake cleaner fluid.
If you are using a multimeter, you should select the range that will put the needle (for the projected reading) in the middle of the scale. If the range setting is too low, the meter will peg for most readings, and if the range setting is too high the needle will barely move for a correct reading. Zero out the ohmmeter. This means holding the meter leads firmly together with one hand and turning the zero adjust knob until the meter reads exactly zero. Now put the leads on the pins of the ABS sensor and read the resistance. It should be very close (within 10 per cent) to the value given in the manual--if not, it is bad and should be replaced.
Get your ohmmeter, rag and brake cleaning fluid ready and near the wheel you are jacking up before you start jacking up the car. You do not want to jack it up and then start looking around for things.
If you are going to go to the store to buy a new ABS sensor, you should put the wheel back on and lower the jack. It sounds like a lot of extra work but you should never leave a car jacked up when you are not around--especially if there are kids or dogs around.