This is just my thoughts.... To not replace spark plugs until the code goes off, it seems like waiting for problems to arise. A few months ago I replaced the factory plugs in our Avalanche (140K), I should have done it at 100K. Now the truck was driving a little slow (but not bad), no codes were going off. 2 of the plugs were fouled (one really bad). After changing the plugs, fuel economy went up by 3.2 Mpg.

Just my opions out loud.