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I agree. You don't have any other codes whatsoever? Given that you have an 02Z, the first thing on the list for a rough idle......are valve springs.
If no other codes I would pull the valve covers and check before you drive or even crank it up. Rough idle is all the warning you might get.
From another post: "my 03 ls6 broke a spring, dropped the valve, ruined the head, put a hole in the piston, and cracked the cylinder. this happened on june 28th of last year, about 2 miles after i filled up with premimum at a country gas station. at first i thought they sold me water, but after about a mile i knew it was much worse, but it was way too late by then."