My ABS light and Brake light stays on. What may the problem be? A dognastic on 2001 Chevrolet Blazer

Mechanic told me my brakes are good, Although the ABS light and Brake Lights stay on. A diagnostic test was recommended to see if the sensor is bad. How much will the diagnostic test cost and the sensor with labor if it is indeed the sensor? If the sensor was recall at 1 time or another will the dealer replace it now? Free of charge?

Asked by for the 2001 Chevrolet Blazer