DETROIT — General Motors took a major step on Tuesday toward boosting its used-vehicle warranties, saying it will offer certified 12 month/12,000-mile 'bumper to bumper' coverage starting September 13.GM's previous warranty covered certified used vehicles for three months or 3,000 miles.'The warranty covers the vehicle from bumper to bumper on any vehicle defect related to materials or workmanship, including all but standard wear-and-tear components on the vehicle,' GM said in a statement. Standard wear-and-tear components are such items as tires and brake pads. The automaker said all body and sheet metal components are warranted against rust-through corrosion for six years or 100,000 miles.The new warranty policy covers Buick, Chevrolet, GMC, Pontiac and Saturn brands. Cadillac, Hummer and Saab certified pre-owned vehicles offer separate warranties: six-year/100,000-mile bumper-to-bumper comprehensive coverage, effective from the vehicle's in-service date.What this means to you: Some good news from GM if you're leaning toward buying a used vehicle this fall. — Anita Lienert, Correspondent