2003 Nissan Frontier with a 4cyl engine, just over 300k.
It vibrates and starts chugging anytime it’s under a small load, like going up a hill. Cruises nice and smooth on level ground or going downhill. Feels like a misfire, but idles perfectly and had new plugs/wires installed a few month ago with no problems.
So there’s no service engine light, and no codes set, but when I plug my scanner into it the code P0400 appears and disappears randomly on the screen but never sets itself in the computer.
My question is, where is a good starting point? Any components known to go bad in the EGR system? And would a bad/stuck EGR valve cause these symptoms?
PS - there is also a code set for the catalytic converter, it was catalyst efficiency below threshold but I don’t remember the exact code. Any way this could be part of the problem, or related in any way?