Thanks @Phantom Z3P0172 is too rich, injectors clogged would be too lean. Could be a stuck open injector. Theoretically, a bad spray pattern could maybe cause inefficient burn and misfiring, which would cause it to be too rich. But you'd have misfiring.
I'd give the map a check. Maybe EGR? Computer thinking it's putting in less EGR gasses into the intake than it really is? That would do it. Has the EGR plate and passages been cleaned?
I might have misfiring. I don't have any audible misfiring, but I'm struggling with another issue of the car violently surging/bucking when I slightly let off of the accelorator (even in VTEC mode, when lean burn is disabled). Part of me wonders whether that surging/bucking might be misfiring. EGR valve has been replaced, EGR plate and passages have been cleaned. And I've followed all of the troubleshooting in the service manual for a P0172.