Originally Posted by dE.fUsEd
Is that the IAB (Intake Air Bypass)?
Yes, but don't let the name mislead you.
All that vacuum hose is doing is drawing against a solenoid valve.
Once that valve is opened, engine vacuum is used (via a diaphragm) to open the butterfly valves and give the intake charge a shorter path.
It's possible you've got a vacuum leak in either the feed hose, or the IAB check valve, but my money is on the aftermarket FPR as the source of your problems.
It first failed HCO and NOX. We didn't know it didn't have a cat.
Put a new cat on, it passed everything but NOX.
So we got the AFPR and turned the fuel pressure up a bit, it then failed HCO and NOX again, but even worse than the original time.
Turned the fuel pressure down quite a bit, and it failed NOX.
It's getting quite frustrating.
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It's probably got some carbon build up, and that plus a hot engine will bump the HC output.
Rather than blindly chase a correct air:fuel ratio with a questionable FPR, I'd just bite the bullet and replace it with an OEM unit.