All of them if you don't want trouble later on down the road.
its best to change all of them , but if you can only access the front plugs change the one which may be causing a code or check engine light
Change them all, it makes no sense to do a half assed tune up. I know they are difficult to change and it costs more money, but you will be better off in the long run.
All or nuthin. would you only put on the left side of you clothes?
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