Trac Off light coming on

My mother-in-law has a 2001 Buick Century. Every once in a while, her trac off light & service engine soon lights will come on & remain lit until she turns off the car. This happened several times without noticing any change in the braking. Now, when she pushes on the brakes, sometimes they sound "gravely" and then a few second later the lights will come on. She's driven more than 40 miles with a mechanic and can't get the problem to duplicate. They say it might be a sensor issue, but that's a $400.00 fix, and she's not ready to pay that if it's more of a real brake issue. Please advise?????