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Incorrect spark plugs torque. This even happens at the factory, most of the time cylinder 3. The plugs have to be torqued accurately in order for them to not back out and not over done to keep from ripping the threads out of the head.
Factory spec is 13 foot pounds. I normally set mine to 20 foot pounds.
When ever replacing the spark plugs. Once every 60 to 90 thousand miles, depending on your driving conditions.