We have had instances of the 5.4 spark plugs breaking when being removed. Ford provides us with a special tool to remove them, however if the rear plugs break it requires removing the head or the if both break remove the engine. Scheduled maintenance suggests inspection or replacement at 100,000 or 160,000 kms depending on model. I reccommend you have the spark plugs inspected before the warranty expires. Either way it is an owner cost to inspect/replace but should they break warranty covers to remove them, saving a potentially costly bill.