All engines covered by this guide utilize timing belts to drive the camshaft from the crankshaft's turning motion and to maintain proper valve timing. Some manufacturer's schedule periodic timing belt replacement to assure optimum engine performance, to make sure the motorist is never stranded should the belt break (as the engine will stop instantly) and for some (manufacturer's with interference motors) to prevent the possibility of severe internal engine damage should the belt break.
Although the 1.5L and 1.8L engines are not listed as an interference motors (it is not listed by the manufacturer as a motor whose valves might contact the pistons if the camshaft was rotated separately from the crankshaft) the first 2 reasons for periodic replacement still apply and the timing belt should be replaced at 60,000 miles (96,000 km). The 1.6L, 2.0L, 2.4L, 3.0L, and 3.5L engines are listed as interference motors, so the timing belt MUST be replaced at 60,000 miles (96,000 km) to avoid severe engine damage if the belt should break.
But whether or not you decide to replace the timing belt in the manufacturers schedule, you would be wise to check it periodically to make sure it has not become damaged or worn. Generally speaking, a severely worn belt may cause engine performance to drop dramatically, but a damaged belt (which could give out suddenly) may not give as much warning. In general, any time the engine timing cover(s) is (are) removed you should inspect the belt for premature parting, severe cracks or missing teeth. Also, an access plug is provided in the upper portion of the timing cover so that camshaft timing can be checked without cover removal. If timing is found to be off, cover removal and further belt inspection or replacement is necessary.