Did you replace the screen while you were in there?

It would be good to know if there was a specific code thrown.

A bad HPOP could be the issue, but the later model HPOPs were generally pretty reliable.

The IPR could just have had lint on it from the oil filter. Did you see the original IPR that they removed? I wonder if you really don't have an issue w/ the STC fitting or dummy plugs/stand pipes - instead of the HPOP.