The timing belt tensioner maintains correct tension on the timing belt, ensuring the crankshaft and camshaft(s) remain synchronised. A failed tensioner allows the belt to slip or jump teeth, causing valve timing errors and potential catastrophic engine damage. The tensioner must be replaced with every timing belt change.
Tensioner failure can be sudden (bearing seizure) or gradual (spring weakening). Both conditions put the timing belt at risk.
The tensioner is always replaced as part of a timing belt kit. Never reuse a tensioner when fitting a new belt.