On my '98 Silverado the mounting surface below the sensor began to corrode, causing the sensor to lift. It only lifted like a millimeter, but enough that the sensors would have issues reading. I never had an ABS light go on, but I had issues at stopping speed where my ABS would go off (pulsating pedal). This was because my front sensors could not get a good reading, didn't match with the rear wheel speed, thus a time ABS should engage. I used a wire brush to clean up the mounting surfaces, worked like a charm! Before doing this I put in a paper towel to the hole to plug it up so debris wouldn't go inside it.