[jayman]

Did the dealership actually check on the condition of the plugs. At +16,000 miles, I'd go ahead and change them anyway

 

[jayman]

I had a 2000 GMC Sierra that I used as a work truck while on a government contract. Had a fill of bad gas from a hole-in-the-wall place, which caused a misfire. The only way to eventually get it to run right was for me to change all the plugs

On the Prius, especially the 2004-2005, it's important to pull the coil packs out to inspect them. That year Prius had a problem with the hood seal, if parked outside rainwater could get into the coil tubes and cause a pretty bad misfire.

It's fairly easy to inspect the Prius coil packs and spark plugs once you unbolt and swing aside the relay box above them. Some Prius owners will pull the coils and use compressed air to dry out the tubes after they wash their engine bay, just to prevent coil problems