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Knock sensor on 2003 Nissan Xterra

so just got this 2003 nissan xterra, 4x4, V6, 5-speed, SWEET truck, but is showing a pending code, knock sensor

1) should i care, because the check-engine light is not on?

2)what exactly does a knock sensor do?

3)the cost to repair (need to take out the intake…EEEEEEEEE…) is around $800! i may be able to do it myself…but…