Try the hose going to your pcv valve. I've got the same problem and I changed the pcv valve, elbow and line and it fixed it. Its going to be something that will only effect one side such as bad gasket on one side or something.

 

It was finally not 100 degrees on a Saturday afternoon and I got a chance to get under the truck. One issue I see right away is there is a loose connection in exhaust where it goes from exhaust manifold in to the pipe that I believe is the start of the catalytic converter..

I attached a picture.. It is hard to tell from picture but

· manifold is at top of picture with the reddish color rust
· the upper flange though it has a rust color that matches the bottom flange appears to be actually part of the manifold.
· the bottom pipe (connected to catalytic converter) has some wiggle to it.
I would guess this is related at least in some way to it running lean. The truck is loud until the exhaust warms up and the check engine light doesnt come back on if I reset it after truck is warm until the time I run the truck.

My question is - that doesn’t looking like a threaded bolt in the top flange - what is the connection hardware that I am missing to pull these guys tight together?

 

I fixed the manifold issue as outlined in the other thread and so far the warning has not come back
https://www.f150forum.com/f6/exhaust...0/#post1921477

 

I got that code too when I had broken manifold studs. It's common.