2001 pathfinder coil

[terry2001]

have been having trouble with my 3.5 running bad ,then running okay .when it thru a code it said that the no 2 cly was not working .changed that coil .the motor runs better than it has for a long time (148000miles)if one coil is bad are they all weak and should they be changed or should I wait till they go bad .they are $100 each. plugs were changed last year.