the abs/trac off light. first start off with jacking each wheel up, then grab the tire at the top, push and pull it firmly, you looking for some movement. should be rather tight feeling. you can also watch the rotor where it meets the brake caliper to see if its moving.
then clean the plug to each hub, and inspect the wires that run along the lca's they get torn up sometimes.
if you cant find any of these free checks to be the problem, its best to pony up and pay to have the abs scanned, or you could be just tossing money at it.
