99 Cougar, P0420, P0430, P1518

1999, 2.5L V6 Mecury Cougar, 103,000 miles

No driveability issues though check engine light has come on and the associated codes are (as in thread title): P0420, P0430, P1518. Seems a serious issue with the emissions & related induction system? A year ago the P1518 code came up and I was able to reset the OBD to get through emissions though the code didn't come back (nor the dreaded check engine light). Well, in fairness, the OBD was reset, but I also had a poke around the induction system with the engine running and the check engine light went off when when the throttle was manually opened. I'm no mechanic, but it almost seemed as though (assuming) the throttle position sensor was lost inasmuch as to where the throttle position actually was. This recent reintroduction seems associated with the turn in the weather to summer time heat. While that's certainly anecdotal, that's about all I know as cause/effect. So any ideas on how to begin trouble shooting (mainly to arm myself with info when dealing with the mechanic--I'm not skilled enough to have a shot at it myself) in addition to ballpark cost potentials?

That manifold has a valve that opens up to allow more air flow into each cylinder at different RPM ranges. It's likely stuck open or closed. It's commaned open or closed by the computer (depending on load, etc). There may be a TSB out about it and you may need to take it in or call and see if there was a reprogramming for it or if it is a physical failure. The 0420and 430 are for your cats. They won't affect drivability, only emissions.