1998 E-150running rich. Run down the road nicely, starts easily, but...

What is your question?

Runs smooth, but black soot is collecting on the quarter-panel and the pipe is sooty black as well. Will a diagnostic uncover the issue? No codes at present. Given the age I thought to change out Oxy sensors for the heck of it. Rumor that could be an issue, but I'll wait on you experts. I seldom work on Fords, except my '68 250 PU.