How to change the spark plugs
It is moderately easy to change the spark plugs on a Liberty, although it is rather tricky because of the gap between the spark plugs.
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Spark plugs and wires
Changing spark plugs is relatively easy. Disconnect the batter and spark plug cables, and remove the old spark plugs. Next replace the spark plugs with new ones, reconnect the spark plug cables and battery.
Have a look at ; www.carbasics.co.uk
This is for the '05 Frontier, but should be what you're looking for. http://www.clubfrontier.org/forums/f23/how-change-spark-plugs-2005-gen-2-frontier-40507/
You will have to remove the intake, and replace the gaskets (they are reuse able but you should have new ones on hand in case they are damaged. The spark plugs installed in the engine are designed to last for 100k miles, unless you have a misfire, then there really no reason to replace them.
Disconnect the battery, remove the spark plug cables and boot, and remove the old spark plugs. Put in the new spark plugs, attach the cables and boots, and connect the battery.
usually every 100000
NGK platinum plugs
On a V6, yes.