Only vehicles with the 3.4L (VIN X) Double Over Head Camshaft (DOHC) engine use a timing belt in conjunction with a timing chain. All other engines use combinations of sprockets and timing chains. Any engine's valve timing is critical to the engine's operation. This is especially the case of any belt-driven over head camshaft engine. In the 3.4L (VIN X) engine, the combination of four camshafts, 24 valves, and tight clearances means that if the belt should slip or otherwise get "out of time", expensive engine damage will result. The timing belt should be inspected for cracks, wear and other damage at 60,000 miles (100,000 km) and then every 15,000 miles (25,000 km). Replace the belt and/or tensioner as needed.