What is a "knock sensor" and what does it do?

Our mechanic wants $528.93 to fix the knock sensor on our 1997 Nissan Maxima. We've experienced some sputtering ... like the car has the hiccups. The breakdown is $216.59 for the knock sensor, $22.50 for the manifold set, $38.52 for the knock sensor sub harness, $2.78 for miscellaneous supplies, and $223.18 for... show more Our mechanic wants $528.93 to fix the knock sensor on our 1997 Nissan Maxima. We've experienced some sputtering ... like the car has the hiccups. The breakdown is $216.59 for the knock sensor, $22.50 for the manifold set, $38.52 for the knock sensor sub harness, $2.78 for miscellaneous supplies, and $223.18 for the labor. Is this necessary, and can anybody in the Tyler, Texas area do this job for less money?
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