I changed mine @ 51K to see if it would help a problem. The dealer wanted almost twice what I paid on line. The ones I put in are iridium but the problem persists so I asked the dealer for the part #. They use the platinum plug by NGK, not the iridium. Neither plug has cured the "hiccup" @ approx. 2700rpm under load (uphill) in 5th & 6th gear. Any advice/ideas will be appreciated.