does this vehicle have an auxillary air pump?
does it work? i seem to recall that the ecu neeeds to see a current draw on the pump circuit before it starts to take notice of the hego sensor, have had this on one a few years ago. i put a bulb in series on the pos circuit of the pump/relay in the fuse box & the emissions came down instantly, much cheaper than a new pump, i'm sure there's something in the archives.
hth.

Steve