why would my clutch go out on a 2012 nissan frontier with 18000 miles
It depends on how well you drive a stick. If you "ride" the clutch you can grind down the plate. Riding means you either leave your foot on the pedal all the time, or when you take off from stopped you control the speed but letting it slip, slip means heat and wear. If you pull heavy loads up steep ramps (like a boat) or commercial weights, you can generate a lot of wear if you don't know what is up. My rule is fully engage the clutch and rest your foot on the dead pedal.
Weak pressure plate would be my first explanation, bad composition on the disc itself, clutch hydraulics not working correctly, or driving around with your foot resting on the clutch pedal.
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