My 2000 Pontiac GP with a 3.8 L, normally aspirated engine came equipped with AC type 41-921 platinum-tipped spark plugs that were supposed to last
100,000 miles. At 41,000 miles I got a "Check Engine Soon" light that turned out to be caused by a #3 cylinder misfire. When I pulled the spark plug on that cylinder I was surprised to see how badly the tip had been erroded. In fact, I was surprised that it would spark at all. I decided to check the other cylinders and found the spark plugs on those cylinders to be in equally-bad condition.
According to AC Delco, the 41-921 platinum-tipped plug has been replaced by a type 41-10 Iridium tipped plug that is supposed to last 100,000 miles. So, if you have 41-921 plugs in your car and they have more than 30,000 miles on them, you may want to pull a couple of them and check their condition. They may not be good for the advertised 100,000 miles of operation.