There have been times when the technicians at National Transmission & Auto Care have found spark plugs seized in the cylinder heads of some of our Houston area customers' vehicles with as little as 60,000 miles on them. The removal of these seized plugs can result in hours of labor by our technicians at National Transmission & Auto Care. There have been rare occasions when National Transmission & Auto Care has needed to remove cylinder heads just to get the spark plugs removed. National Transmission & Auto Care suggests that in order for our customers in the Houston are to maintain peak performance for their vehicles, that their spark plugs should not be run longer than 30,000 miles. National Transmission & Auto Care suggest that consumers should take those 100,000 mile spark plug marketing claims of manufacturers with a grain of salt.