Every year I change my plugs... Period...
Yah, it costs money, and yah, the plugs are still good...
But I accomplish two things by doing this:
1: I always have good condition plugs
2: The plugs are a hell of a lot easier to change every year than if I were to change them every couple of years... The longer they sit, the harder they are to remove... And breaking a plug while it's still screwed in sucks...
Since I haven't had my Catera very long, I haven't done much service work to it yet... But it's nearing the 55k mile mark, and as soon as it does, I'm doing my "tune-up"
-Elo