The timing belt tensioner is what applies pressure to the timing belt, keeping it tight and running smoothly. When something goes wrong, it is often the tensioner that is the cause and not the timing belt itself.
But to prevent the timing belt from failing, the timing belt, tensioner, and the idler pulleys should be replaced every 50 to 70,000 miles.
But be sure to ask your service advisor what your recommended service interval is.
If the tensioner breaks, the timing belt can slip or break too and the split second synchronization of engine parts is lost and the damage could be catastrophic.
A broken timing belt can cause the pistons to collide with the valves,
potentially bending or breaking them, as well as the piston, cylinder and crank-shaft.
From a safety standpoint, there is slight safety risk, because if the car breaks down, it could end up stranded.
But there is no environmental impact of timing belt tensioner replacement.