IAT sensor connection to P042 code?

Hey all,

Another post regarding everybody's favorite subject, the P0420 code. As we've gotten into Spring, here near Durham, NC, I've noticed that on warm days, approx above 70 deg F, the Readiness test comes up with no incompletes, and no Cat code, just all pretty green check marks. The engine runs great all day, peppy and responsive, the way it used to. As soon as the ambient temp drops later in the day, like maybe below 70, the pending Cat code shows up, and the engine goes back to running like its dragging an anchor.

I've also noticed the the IAT reading always seems to be way high, a good 20-25 deg above the ambient temp (I know that its going to be warm in the engine compartment, but this is even at startup or when reading right away after moving.

I wonder if the IAT sensor be out of whack, causing the ECT to come up with a faulty /false Cat code? Or what else might be happening?

If anybody has any professional experience or deeper knowledge of these systems than me, I'd appreciate whatever input/suggestions you be willing to share.

Thanks in advance