did you reset the ecu/ecm to clear the codes? see if the same codes do come back again... then suspect a wiring problem for the knock sensor before condemning the sensor itself to be bad
click on the link i provided:
http://www.nicoclub.com/FSM/quest/1997_Quest/ec.pdf
goto page EC-149 to follow directions on testing the knock sensor and the possible open circuit in the wiring
{on that page you'll know that the code will not set the check engine light} but to check the wiring , electrical plug, and then the sensor...
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as for the fuel gauge problem, you'll need to check the power and ground circuits...may be an open in the wiring from the sending unit to various body grounds
click on the link i provided:
http://www.nicoclub.com/FSM/quest/1997_Quest/el.pdf
goto section EL-95 & follow the directions there