Mystery with gen 2 prius

2008 Prius with 200K. Last 4 months: rough cold start, significant drop in mpg, carbon buildup in tailpipe. No check engine light. O'Reilly computer analysis shows codes 300, 301, 302 and "Frequent random misfires cylinders 1 & 2." Sparkplugs changed. No change. Injector coils switched around. No change. I'm not very handy. Don't want to spend a fortune on "fishing expeditions" either with diagnostics or repairs/replacements without an actual diagnosis. Has been a great car. Any ideas/suggestions?

 

Thanks for the replies. The plugs are 3 months old and still look fine.. Switching the coils around didn’t change the fact that cylinders 1 and 2 are still the ones misfiring. So that would seem to rule out anything to do with the coils being the problem.

Yes the plugs are oem from the dealer.

 

As I said in my original post the computer analysis always shows 300, 301, 302 “frequent random misfires cylinders 1 and 2.” Replacing the plugs and reanalyzing didn’t change that and switching around the injector coils and reanalyzing also didn’t change that. So it seems like the problem has to be with both cylinders 1 and 2.

 

I suppose another possibility is that the fuel injectors on cylinders 1 & 2 both malfunctioned at the same time while the injectors on cylinders 3 & 4 haven't malfunctioned. But doesn't that seem like an unlikely coincidence? Isn't it more likely that there is a problem between cylinders 1 & 2?

 

That would certainly be one explanation. I'm starting to think about getting a JDM engine replacement.