High Nox but truck runs great. No DTC's.

2004 F150 5.4 3V. Truck runs great, no noticeable drivability issues, no c.e.l. failed emissions for high nox. I bought this truck with bad VCT cam gears. The previous owner drove it in that condition for a couple of months. I found the cats were melted closed after replacing the entire timing set (CLOYES), and oil pump (MELLING). After completing the above work I disconnected the cats and truck ran fine. Replaced cats with a new set off of eBay and new O.E.M. oxygen sensors. No codes, runs great, no driveability issues, ran it through emissions and failed for high Nox. Fuel pressure at 39-40 psi, smoked test proved inconclusive.

I prayed diligently the Lord to guide me in this and he said ask for help. "Knock and it shall be opened, seek and ye shall find"

I will post fuel trims at idle and at 2k rpm soon.

Thank you Dan for this site and may God Bless all whom come here...

Hey Olddognewtricks! Welcome to the forums.

It's tough not to be suspicious of the eBay cats. If I were you, I'd focus on scan data during a good long test drive. Fuel trims, engine temperature, and VCT performance. Make sure the fuel trims are within 10% in Closed Loop, that the engine isn't overheating, and that the VCT desired angle meets desired. I know there's no codes, but that doesn't mean you can't have a marginal failure that impacts the catalyst's ability to do its job.

If all of the above checks out, then I'd suspect the cats.

I had an Nissan D21 4cyl that went through head gaskets about every 20k miles almost like clock work... happened twice that my smog check was due around the 20k period and the truck failed smog with high NOx readings. Last time it happened I started watching my cylinder pressures and sure enough, 1 & 2 dropped and I failed smog with high NOx. Week after failing smog, head gasket blew between 1 & 2. Truck always ran great no codes but started loosing power towards the end.