check engine codes P0303/P0306 cylinder 3/6 misfire How do I repair this?

Hello marcjames, First, I would like to thank you for reaching out to us! Diagnosing misfires can be very tricky, because the ignition system is made up of several different components that all must work together perfectly, every time. Best case scenario you just need new plugs and coils, worst case scenario the engine has lost compression to the point that the air/fuel mixture in the cylinder is not firing properly. The best way to handle this is to have the issue looked at by a certified technician, who will then be able to tell you exactly what is going on and avoid your money being wasted on ineffectual repairs. Measure twice, cut once! We would be more than happy to take a look at it for you and figure it out. The Siennas are great vehicles and I am confident we can get you fixed up in no time!