A worn tensioner will give the timing chain too much slack, and this may lead to a slapping noise, especially when first starting the engine.
A worn tensioner may also cause the check engine light to come on, and the engine may run rough, hesitate or stall.
Common misdiagnoses
The sound of timing chain slap is similar to a tapping lifter at startup, so many times they are not replaced because the sound goes away after a few moments.
Best practices
When the timing chain tensioner has failed, there may be damage to the timing chain guide rails or engine front cover, so the technician may recommend further inspection or changing the oil to check for debris.
For engines with more than one timing chain tensioner, they should be replaced as a set.