The truck has 64000 miles. We were discussing the 60k service recently, and I'm working on it, piecemeal. Two plug wires on the right bank look a little shabby (boots look white and "frosty"). I am planning on replacing them soon (the whole set).
It's been my experience that a misfire will cause a low CO reading and high HC, similar to a too-lean mixture. None of my previous experience has been with Landrovers, and I quit working on cars for a living in 1994, so the OBD II and many of the newer electronic systems are new to me.
Maybe if the HC from the misfire gets burned in the cat, the downstream O2 sensor will pick it up the extra CO as "rich"?
If the mixture is rich on both banks, the only component I can think of that's common to both banks is the MAF sensor.
Thanks for the advice, I enjoy these discussions.